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All trademarks and registered trademarks are the property of their respective owners Contents About ................................................................................................................................................................................... 1 Chapter 1: Getting started with Perl Language ............................................................................................ 2 Section 1.1: Getting started with Perl ............................................................................................................................ 2 Chapter 2: Comments ................................................................................................................................................. 4 Section 2.1: Single-line comments ................................................................................................................................ 4 Section 2.2: Multi-line comments ................................................................................................................................. 4 Chapter 3: Variables .................................................................................................................................................... 5 Section 3.1: Scalars ........................................................................................................................................................ 5 Section 3.2: Array References ...................................................................................................................................... 5 Section 3.3: Scalar References ..................................................................................................................................... 6 Section 3.4: Arrays ......................................................................................................................................................... 7 Section 3.5: Typeglobs, typeglob refs, filehandles and constants ........................................................................... 8 Section 3.6: Sigils ............................................................................................................................................................ 9 Section 3.7: Hash References ..................................................................................................................................... 11 Section 3.8: Hashes ..................................................................................................................................................... 12 Chapter 4: Interpolation in Perl .......................................................................................................................... 15 Section 4.1: What is interpolated ................................................................................................................................ 15 Section 4.2: Basic interpolation .................................................................................................................................. 16 Chapter 5: True and false ...................................................................................................................................... 18 Section 5.1: List of true and false values ................................................................................................................... 18 Chapter 6: Dates and Time .................................................................................................................................... 19 Section 6.1: Date formatting ....................................................................................................................................... 19 Section 6.2: Create new DateTime ............................................................................................................................ 19 Section 6.3: Working with elements of datetime ..................................................................................................... 19 Section 6.4: Calculate code execution time .............................................................................................................. 20 Chapter 7: Control Statements ........................................................................................................................... 21 Section 7.1: Conditionals .............................................................................................................................................. 21 Section 7.2: Loops ....................................................................................................................................................... 21 Chapter 8: Subroutines ............................................................................................................................................ 23 Section 8.1: Creating subroutines ............................................................................................................................... 23 Section 8.2: Subroutines ............................................................................................................................................. 24 Section 8.3: Subroutine arguments are passed by reference (except those in signatures) ............................... 25 Chapter 9: Debug Output ....................................................................................................................................... 27 Section 9.1: Dumping with Style ................................................................................................................................. 27 Section 9.2: Dumping data-structures ...................................................................................................................... 28 Section 9.3: Data::Show ............................................................................................................................................... 28 Section 9.4: Dumping array list .................................................................................................................................. 29 Chapter 10: Lists .......................................................................................................................................................... 31 Section 10.1: Array as list ............................................................................................................................................. 31 Section 10.2: Assigning a list to a hash ...................................................................................................................... 31 Section 10.3: Lists can be passed into subroutines .................................................................................................. 31 Section 10.4: Return list from subroutine .................................................................................................................. 32 Section 10.5: Hash as list ............................................................................................................................................. 33 Section 10.6: Using arrayref to pass array to sub ................................................................................................... 33 Chapter 11: Sorting ..................................................................................................................................................... 34 Section 11.1: Basic Lexical Sort .................................................................................................................................... 34 Section 11.2: The Schwartzian Transform ................................................................................................................. 34 Section 11.3: Case Insensitive Sort .............................................................................................................................. 35 Section 11.4: Numeric Sort ........................................................................................................................................... 35 Section 11.5: Reverse Sort ........................................................................................................................................... 35 Chapter 12: File I/O (reading and writing files) ........................................................................................... 36 Section 12.1: Opening A FileHandle for Reading ...................................................................................................... 36 Section 12.2: Reading from a file ............................................................................................................................... 36 Section 12.3: Write to a file .......................................................................................................................................... 37 Section 12.4: "use autodie" and you won't need to check file open/close failures .............................................. 37 Section 12.5: Rewind a filehandle ............................................................................................................................... 38 Section 12.6: Reading and Writing gzip compressed files ....................................................................................... 38 Section 12.7: Setting the default Encoding for IO ..................................................................................................... 39 Chapter 13: Reading a file's content into a variable ................................................................................ 40 Section 13.1: Path::Tiny ................................................................................................................................................. 40 Section 13.2: The manual way .................................................................................................................................... 40 Section 13.3: File::Slurp ................................................................................................................................................. 40 Section 13.4: File::Slurper ............................................................................................................................................. 41 Section 13.5: Slurping a file into an array variable ................................................................................................... 41 Section 13.6: Slurp file in one-liner .............................................................................................................................. 41 Chapter 14: Strings and quoting methods .................................................................................................... 42 Section 14.1: String Literal Quoting ............................................................................................................................. 42 Section 14.2: Double-quoting ...................................................................................................................................... 42 Section 14.3: Heredocs ................................................................................................................................................ 43 Section 14.4: Removing trailing newlines .................................................................................................................. 44 Chapter 15: Split a string on unquoted separators ................................................................................... 46 Section 15.1: parse_line() ............................................................................................................................................ 46 Section 15.2: Text::CSV or Text::CSV_XS .................................................................................................................... 46 Chapter 16: Object-oriented Perl ........................................................................................................................ 47 Section 16.1: Defining classes in modern Perl ........................................................................................................... 47 Section 16.2: Creating Objects .................................................................................................................................... 47 Section 16.3: Defining Classes .................................................................................................................................... 48 Section 16.4: Inheritance and methods resolution ................................................................................................... 49 Section 16.5: Class and Object Methods .................................................................................................................... 51 Section 16.6: Roles ....................................................................................................................................................... 52 Chapter 17: Exception handling ........................................................................................................................... 54 Section 17.1: eval and die ............................................................................................................................................ 54 Chapter 18: Regular Expressions ........................................................................................................................ 55 Section 18.1: Replace a string using regular expressions ........................................................................................ 55 Section 18.2: Matching strings .................................................................................................................................... 55 Section 18.3: Parsing a string with a regex ............................................................................................................... 55 Section 18.4: Usage of \Q and \E in pattern matching ........................................................................................... 56 Chapter 19: XML Parsing .......................................................................................................................................... 57 Section 19.1: Parsing with XML::Twig .......................................................................................................................... 57 Section 19.2: Consuming XML with XML::Rabbit ....................................................................................................... 58 Section 19.3: Parsing with XML::LibXML ..................................................................................................................... 60 Chapter 20: Unicode .................................................................................................................................................. 62 Section 20.1: The utf8 pragma: using Unicode in your sources ............................................................................. 62 Section 20.2: Handling invalid UTF-8 ........................................................................................................................ 62 Section 20.3: Command line switches for one-liners ............................................................................................... 63 Section 20.4: Standard I/O ......................................................................................................................................... 64 Section 20.5: File handles ........................................................................................................................................... 64 Section 20.6: Create filenames .................................................................................................................................. 65 Section 20.7: Read filenames ..................................................................................................................................... 66 Chapter 21: Perl one-liners ..................................................................................................................................... 68 Section 21.1: Upload file into mojolicious ................................................................................................................... 68 Section 21.2: Execute some Perl code from command line .................................................................................... 68 Section 21.3: Using double-quoted strings in Windows one-liners ......................................................................... 68 Section 21.4: Print lines matching a pattern (PCRE grep) ....................................................................................... 68 Section 21.5: Replace a substring with another (PCRE sed) ................................................................................... 69 Section 21.6: Print only certain fields ......................................................................................................................... 69 Section 21.7: Print lines 5 to 10 .................................................................................................................................... 69 Section 21.8: Edit file in-place ..................................................................................................................................... 69 Section 21.9: Reading the whole file as a string ....................................................................................................... 69 Chapter 22: Randomness ........................................................................................................................................ 70 Section 22.1: Accessing an array element at random ............................................................................................. 70 Section 22.2: Generate a random integer between 0 and 9 .................................................................................. 70 Chapter 23: Special variables ............................................................................................................................... 71 Section 23.1: Special variables in perl: ....................................................................................................................... 71 Chapter 24: Packages and modules ................................................................................................................. 72 Section 24.1: Using a module ..................................................................................................................................... 72 Section 24.2: Using a module inside a directory ...................................................................................................... 72 Section 24.3: Loading a module at runtime .............................................................................................................. 73 Section 24.4: CPAN.pm ................................................................................................................................................ 73 Section 24.5: List all installed modules ...................................................................................................................... 74 Section 24.6: Executing the contents of another file ............................................................................................... 74 Chapter 25: Install Perl modules via CPAN .................................................................................................... 75 Section 25.1: cpanminus, the lightweight configuration-free replacement for cpan ........................................... 75 Section 25.2: Installing modules manually ............................................................................................................... 75 Section 25.3: Run Perl CPAN in your terminal (Mac and Linux) or command prompt (Windows) .................... 76 Chapter 26: Easy way to check installed modules on Mac and Ubuntu ......................................... 78 Section 26.1: Use perldoc to check the Perl package install path .......................................................................... 78 Section 26.2: Check installed perl modules via terminal ......................................................................................... 78 Section 26.3: How to check Perl corelist modules .................................................................................................... 78 Chapter 27: Pack and unpack .............................................................................................................................. 79 Section 27.1: Manually Converting C Structs to Pack Syntax .................................................................................. 79 Section 27.2: Constructing an IPv4 header ............................................................................................................... 80 Chapter 28: Perl commands for Windows Excel with Win32::OLE module .................................... 82 Section 28.1: Opening and Saving Excel/Workbooks .............................................................................................. 82 Section 28.2: Manipulation of Worksheets ............................................................................................................... 82 Section 28.3: Manipulation of cells ............................................................................................................................ 83 Section 28.4: Manipulation of Rows / Columns ....................................................................................................... 84 Chapter 29: Simple interaction with database via DBI module .......................................................... 85 Section 29.1: DBI module ............................................................................................................................................ 85 Chapter 30: Perl Testing ......................................................................................................................................... 87 Section 30.1: Perl Unit Testing Example .................................................................................................................... 87 Chapter 31: Dancer ..................................................................................................................................................... 89 Section 31.1: Easiest example ...................................................................................................................................... 89 Chapter 32: Attributed Text ................................................................................................................................... 90 Section 32.1: Printing colored Text ............................................................................................................................. 90 Chapter 33: GUI Applications in Perl .................................................................................................................. 91 Section 33.1: GTK Application ..................................................................................................................................... 91 Chapter 34: Memory usage optimization ...................................................................................................... 92 Section 34.1: Reading files: foreach vs. while ............................................................................................................ 92 Section 34.2: Processing long lists ............................................................................................................................. 92 Chapter 35: Perl script debugging ..................................................................................................................... 93 Section 35.1: Run script in debug mode .................................................................................................................... 93 Section 35.2: Use a nonstandard debugger ............................................................................................................. 93 Chapter 36: Perlbrew ................................................................................................................................................ 94 Section 36.1: Setup perlbrew for the first time .......................................................................................................... 94 Chapter 37: Installation of Perl ........................................................................................................................... 95 Section 37.1: Linux ........................................................................................................................................................ 95 Section 37.2: OS X ........................................................................................................................................................ 95 Section 37.3: Windows ................................................................................................................................................. 96 Chapter 38: Compile Perl cpan module sapnwrfc from source code .............................................. 97 Section 38.1: Simple example to test the RFC connection ....................................................................................... 97 Chapter 39: Best Practices ..................................................................................................................................... 98 Section 39.1: Using Perl::Critic ..................................................................................................................................... 98 Credits ............................................................................................................................................................................ 102 You may also like ...................................................................................................................................................... 104 About Please feel free to share this PDF with anyone for free, latest version of this book can be downloaded from: https://goalkicker.com/PerlBook This Perl® Notes for Professionals book is compiled from Stack Overflow Documentation, the content is written by the beautiful people at Stack Overflow. Text content is released under Creative Commons BY-SA, see credits at the end of this book whom contributed to the various chapters. Images may be copyright of their respective owners unless otherwise specified This is an unofficial free book created for educational purposes and is not affiliated with official Perl® group(s) or company(s) nor Stack Overflow. All trademarks and registered trademarks are the property of their respective company owners The information presented in this book is not guaranteed to be correct nor accurate, use at your own risk Please send feedback and corrections to [email protected] GoalKicker.com – Perl® Notes for Professionals 1 Chapter 1: Getting started with Perl Language Version Release Notes Release Date 1.000 1987-12-18 2.000 1988-06-05 3.000 1989-10-18 4.000 1991-03-21 5.000 1994-10-17 5.001 1995-05-13 5.002 1996-02-29 5.003 1996-06-25 5.004 perl5004delta 1997-05-15 5.005 perl5005delta 1998-07-22 5.6.0 perl56delta 2000-03-22 5.8.0 perl58delta 2002-07-18 perl581delta, perl582delta, perl583delta, 5.8.8 perl584delta, perl585delta, perl586delta, perl587delta, perl588delta 2006-02-01 5.10.0 perl5100delta 2007-12-18 5.12.0 perl5120delta 2010-04-12 5.14.0 perl5140delta 2011-05-14 5.16.0 perl5160delta 2012-05-20 5.18.0 perl5180delta 2013-05-18 5.20.0 perl5200delta 2014-05-27 5.22.0 perl5220delta 2015-06-01 5.24.0 perl5240delta 2016-05-09 5.26.0 perl5260delta 2017-05-30 Section 1.1: Getting started with Perl Perl tries to do what you mean: print "Hello World\n"; The two tricky bits are the semicolon at the end of the line and the \n, which adds a newline (line feed). If you have a relatively new version of perl, you can use say instead of print to have the carriage return added automatically: Version ≥ 5.10.0 use feature 'say'; say "Hello World"; The say feature is also enabled automatically with a use v5.10 (or higher) declaration: use v5.10; GoalKicker.com – Perl® Notes for Professionals 2 say "Hello World"; It's pretty common to just use perl on the command line using the -e option: $ perl -e 'print "Hello World\n"' Hello World Adding the -l option is one way to print newlines automatically: $ perl -le 'print "Hello World"' Hello World Version ≥ 5.10.0 If you want to enable new features, use the -E option instead: $ perl -E 'say "Hello World"' Hello World You can also, of course, save the script in a file. Just remove the -e command line option and use the filename of the script: perl script.pl. For programs longer than a line, it's wise to turn on a couple of options: use strict; use warnings; print "Hello World\n"; There's no real disadvantage other than making the code slightly longer. In exchange, the strict pragma prevents you from using code that is potentially unsafe and warnings notifies you of many common errors. Notice the line-ending semicolon is optional for the last line, but is a good idea in case you later add to the end of your code. For more options how to run Perl, see perlrun or type perldoc perlrun at a command prompt. For a more detailed introduction to Perl, see perlintro or type perldoc perlintro at a command prompt. For a quirky interactive tutorial, Try Perl. GoalKicker.com – Perl® Notes for Professionals 3 Chapter 2: Comments Section 2.1: Single-line comments Single-line comments begin with a pound sign # and go to the end of the line: # This is a comment my $foo = "bar"; # This is also a comment Section 2.2: Multi-line comments Multi-line comments start with = and with the =cut statement. These are special comments called POD (Plain Old Documentation). Any text between the markers will be commented out: =begin comment This is another comment. And it spans multiple lines! =end comment =cut GoalKicker.com – Perl® Notes for Professionals 4 Chapter 3: Variables Section 3.1: Scalars Scalars are Perl's most basic data type. They're marked with the sigil $ and hold a single value of one of three types: a number (3, 42, 3.141, etc.) a string ('hi', "abc", etc.) a reference to a variable (see other examples). my $integer = 3; # number my $string = "Hello World"; # string my $reference = \$string; # reference to $string Perl converts between numbers and strings on the fly, based on what a particular operator expects. my $number = '41'; # string '41' my $meaning = $number + 1; # number 42 my $sadness = '20 apples'; # string '20 apples' my $danger = $sadness * 2; # number '40', raises warning When converting a string into a number, Perl takes as many digits from the front of a string as it can – hence why 20 apples is converted into 20 in the last line. Based on whether you want to treat the contents of a scalar as a string or a number, you need to use different operators. Do not mix them. # String comparison # Number comparison 'Potato' eq 'Potato'; 42 == 42; 'Potato' ne 'Pomato'; 42 != 24; 'Camel' lt 'Potato'; 41 < 42; 'Zombie' gt 'Potato'; 43 > 42; # String concatenation # Number summation 'Banana' . 'phone'; 23 + 19; # String repetition # Number multiplication 'nan' x 3; 6 * 7; Attempting to use string operations on numbers will not raise warnings; attempting to use number operations on non-numeric strings will. Do be aware that some non-digit strings such as 'inf', 'nan', '0 but true' count as numbers. Section 3.2: Array References Array References are scalars ($) which refer to Arrays. my @array = ("Hello"); # Creating array, assigning value from a list my $array_reference = \@array; These can be created more short-hand as follows: my $other_array_reference = ["Hello"]; Modifying / Using array references require dereferencing them first. GoalKicker.com – Perl® Notes for Professionals 5 my @contents = @{ $array_reference }; # Prefix notation my @contents = @$array_reference; # Braces can be left out Version ≥ 5.24.0 New postfix dereference syntax, available by default from v5.24 use v5.24; my @contents = $array_reference->@*; # New postfix notation When accessing an arrayref's contents by index you can use the -> syntactical sugar. my @array = qw(one two three); my $arrayref = [ qw(one two three) ] my $one = $array[0]; my $one = $arrayref->[0]; Unlike arrays, arrayrefs can be nested: my @array = ( (1, 0), (0, 1) ) # ONE array of FOUR elements: (1, 0, 0, 1) my @matrix = ( [1, 0], [0, 1] ) # an array of two arrayrefs my $matrix = [ [0, 1], [1, 0] ] # an arrayref of arrayrefs # There is no namespace conflict between scalars, arrays and hashes # so @matrix and $matrix _both_ exist at this point and hold different values. my @diagonal_1 = ($matrix[0]->[1], $matrix[1]->[0]) # uses @matrix my @diagonal_2 = ($matrix->[0]->[1], $matrix->[1]->[0]) # uses $matrix # Since chained []- and {}-access can only happen on references, you can # omit some of those arrows. my $corner_1 = $matrix[0][1]; # uses @matrix; my $corner_2 = $matrix->[0][1]; # uses $matrix; When used as Boolean, references are always true. Section 3.3: Scalar References A reference is a scalar variable (one prefixed by $ ) which “refers to” some other data. my $value = "Hello"; my $reference = \$value; print $value; # => Hello print $reference; # => SCALAR(0x2683310) To get the referred-to data, you de-reference it. say ${$reference}; # Explicit prefix syntax say $$reference; # The braces can be left out (confusing) Version ≥ 5.24.0 New postfix dereference syntax, available by default from v5.24 use v5.24; say $reference->$*; # New postfix notation This "de-referenced value" can then be changed like it was the original variable. ${$reference} =~ s/Hello/World/; print ${$reference}; # => World print $value; # => World GoalKicker.com – Perl® Notes for Professionals 6 A reference is always truthy – even if the value it refers to is falsy (like 0 or ""). You may want a Scalar Reference If: You want to pass a string to a function, and have it modify that string for you without it being a return value. You wish to explicitly avoid Perl implicitly copying the contents of a large string at some point in your function passing ( especially relevant on older Perls without copy-on-write strings ) You wish to disambiguate string-like values with specific meaning, from strings that convey content, for example: Disambiguate a file name from file content Disambiguate returned content from a returned error string You wish to implement a lightweight inside out object model, where objects handed to calling code don't carry user visible metadata: our %objects; my $next_id = 0; sub new { my $object_id = $next_id++; $objects{ $object_id } = { ... }; # Assign data for object my $ref = \$object_id; return bless( $ref, "MyClass" ); } Section 3.4: Arrays Arrays store an ordered sequence of values. You can access the contents by index, or iterate over them. The values will stay in the order you filled them in. my @numbers_to_ten = (1,2,3,4,5,6,7,8,9,10); # More conveniently: (1..10) my @chars_of_hello = ('h','e','l','l','o'); my @word_list = ('Hello','World'); # Note the sigil: access an @array item with $array[index] my $second_char_of_hello = $chars_of_hello[1]; # 'e' # Use negative indices to count from the end (with -1 being last) my $last_char_of_hello = $chars_of_hello[-1]; # Assign an array to a scalar to get the length of the array my $length_of_array = @chars_of_hello; # 5 # You can use $# to get the last index of an array, and confuse Stack Overflow my $last_index_of_array = $#chars_of_hello; # 4 # You can also access multiple elements of an array at the same time # This is called "array slice" # Since this returns multiple values, the sigil to use here on the RHS is @ my @some_chars_of_hello = @chars_of_hello[1..3]; # ('H', 'e', 'l') my @out_of_order_chars = @chars_of_hello[1,4,2]; # ('e', 'o', 'l') # In Python you can say array[1:-1] to get all elements but first and last # Not so in Perl: (1..-1) is an empty list. Use $# instead my @empty_list = @chars_of_hello[1..-1]; # () my @inner_chars_of_hello = @chars_of_hello[1..$#chars_of_hello-1]; # ('e','l','l') GoalKicker.com – Perl® Notes for Professionals 7 # Access beyond the end of the array yields undef, not an error my $undef = $chars_of_hello[6]; # undef Arrays are mutable: use utf8; # necessary because this snippet is utf-8 $chars_of_hello[1] = 'u'; # ('h','u','l','l','o') push @chars_of_hello, ('!', '!'); # ('h','u','l','l','o','!','!') pop @chars_of_hello; # ('h','u','l','l','o','!') shift @chars_of_hello; # ('u','l','l','o','!') unshift @chars_of_hello, ('¡', 'H'); # ('¡','H','u','l','l','o','!') @chars_of_hello[2..5] = ('O','L','A'); # ('¡','H','O','L','A',undef,'!') whoops! delete $chars_of_hello[-2]; # ('¡','H','O','L','A', '!') # Setting elements beyond the end of an array does not result in an error # The array is extended with undef's as necessary. This is "autovivification." my @array; # () my @array[3] = 'x'; # (undef, undef, undef, 'x') Finally, you can loop over the contents of an array: use v5.10; # necessary for 'say' for my $number (@numbers_to_ten) { say $number ** 2; } When used as booleans, arrays are true if they are not empty. Section 3.5: Typeglobs, typeglob refs, filehandles and constants A typeglob *foo holds references to the contents of global variables with that name: $foo, @foo, $foo, &foo, etc. You can access it like an hash and assign to manipulate the symbol tables directly (evil!). use v5.10; # necessary for say our $foo = "foo"; our $bar; say ref *foo{SCALAR}; # SCALAR say ${ *foo{SCALAR} }; # bar *bar = *foo; say $bar; # bar $bar = 'egg'; say $foo; # egg Typeglobs are more commonly handled when dealing with files. open, for example, produces a reference to a typeglob when asked to create a non-global filehandle: use v5.10; # necessary for say open(my $log, '> utf-8', '/tmp/log') or die $!; # open for writing with encoding say $log 'Log opened'; # You can dereference this globref, but it's not very useful. say ref $log; # GLOB say (*{$log}->{IO} // 'undef'); # undef close $log or die $!; GoalKicker.com – Perl® Notes for Professionals 8 Typeglobs can also be used to make global read-only variables, though use constant is in broader use. # Global constant creation *TRUE = \('1'); our $TRUE; say $TRUE; # 1 $TRUE = ''; # dies, "Modification of a read-only value attempted" # use constant instead defines a parameterless function, therefore it's not global, # can be used without sigils, can be imported, but does not interpolate easily. use constant (FALSE => 0); say FALSE; # 0 say &FALSE; # 0 say "${\FALSE}"; # 0 (ugh) say *FALSE{CODE}; # CODE(0xMA1DBABE) # Of course, neither is truly constant when you can manipulate the symbol table... *TRUE = \(''); use constant (EVIL => 1); *FALSE = *EVIL; Section 3.6: Sigils Perl has a number of sigils: $scalar = 1; # individual value @array = ( 1, 2, 3, 4, 5 ); # sequence of values %hash = ('it', 'ciao', 'en', 'hello', 'fr', 'salut'); # unordered key-value pairs &function('arguments'); # subroutine *typeglob; # symbol table entry These look like sigils, but aren't: \@array; # \ returns the reference of what's on the right (so, a reference to @array) $#array; # this is the index of the last element of @array You can use braces after the sigil if you should be so inclined. Occasionally, this improves readability. say ${value} = 5; While you use different sigils to define variables of different types, the same variable can be accessed in different ways based on what sigils you use. %hash; # we use % because we are looking at an entire hash $hash{it}; # we want a single value, however, that's singular, so we use $ $array[0]; # likewise for an array. notice the change in brackets. @array[0,3]; # we want multiple values of an array, so we instead use @ @hash{'it','en'}; # similarly for hashes (this gives the values: 'ciao', 'hello') %hash{'it','fr'}; # we want an hash with just some of the keys, so we use % # (this gives key-value pairs: 'it', 'ciao', 'fr', 'salut') This is especially true of references. In order to use a referenced value you can combine sigils together. my @array = 1..5; # This is an array my $reference_to_an_array = \@array; # A reference to an array is a singular value push @array, 6; # push expects an array push @$reference_to_an_array, 7; # the @ sigil means what's on the right is an array # and what's on the right is $reference_to_an_array GoalKicker.com – Perl® Notes for Professionals 9 # hence: first a @, then a $ Here's a perhaps less confusing way to think about it. As we saw earlier, you can use braces to wrap what's on the right of a sigil. So you can think of @{} as something that takes an array reference and gives you the referenced array. # pop does not like array references pop $reference_to_an_array; # ERROR in Perl 5.20+ # but if we use @{}, then... pop @{ $reference_to_an_array }; # this works! As it turns out, @{} actually accepts an expression: my $values = undef; say pop @{ $values }; # ERROR: can't use undef as an array reference say pop @{ $values // [5] } # undef // [5] gives [5], so this prints 5 ...and the same trick works for other sigils, too. # This is not an example of good Perl. It is merely a demonstration of this language feature my $hashref = undef; for my $key ( %{ $hashref // {} } ) { "This doesn't crash"; } ...but if the "argument" to a sigil is simple, you can leave the braces away. say $$scalar_reference; say pop @$array_reference; for keys (%$hash_reference) { ... }; Things can get excessively extravagant. This works, but please Perl responsibly. my %hash = (it => 'ciao', en => 'hi', fr => 'salut'); my $reference = \%hash; my $reference_to_a_reference = \$reference; my $italian = $hash{it}; # Direct access my @greets = @$reference{'it', 'en'}; # Dereference, then access as array my %subhash = %$$reference_to_a_reference{'en', 'fr'} # Dereference ×2 then access as hash For most normal use, you can just use subroutine names without a sigil. (Variables without a sigil are typically called "barewords".) The & sigil is only useful in a limited number of cases. Making a reference to a subroutine: sub many_bars { 'bar' x $_[0] } my $reference = \&many_bars; say $reference->(3); # barbarbar Calling a function ignoring its prototype. Combined with goto, as a slightly weird function call that has the current call frame replaced with the caller. Think the linux exec() API call, but for functions. GoalKicker.com – Perl® Notes for Professionals 10 Section 3.7: Hash References Hash references are scalars which contain a pointer to the memory location containing the data of a hash. Because the scalar points directly to the hash itself, when it is passed to a subroutine, changes made to the hash are not local to the subroutine as with a regular hash, but instead are global. First, let's examine what happens when you pass a normal hash to a subroutine and modify it within there: use strict; use warnings; use Data::Dumper; sub modify { my %hash = @_; $hash{new_value} = 2; print Dumper("Within the subroutine"); print Dumper(\%hash); return; } my %example_hash = ( old_value => 1, ); modify(%example_hash); print Dumper("After exiting the subroutine"); print Dumper(\%example_hash); Which results in: $VAR1 = 'Within the subroutine'; $VAR1 = { 'new_value' => 2, 'old_value' => 1 }; $VAR1 = 'After exiting the subroutine'; $VAR1 = { 'old_value' => 1 }; Notice that after we exit the subroutine, the hash remains unaltered; all changes to it were local to the modify subroutine, because we passed a copy of the hash, not the hash itself. In comparison, when you pass a hashref, you are passing the address to the original hash, so any changes made within the subroutine will be made to the original hash: use strict; use warnings; use Data::Dumper; sub modify { my $hashref = shift; GoalKicker.com – Perl® Notes for Professionals 11 # De-reference the hash to add a new value $hashref->{new_value} = 2; print Dumper("Within the subroutine"); print Dumper($hashref); return; } # Create a hashref my $example_ref = { old_value => 1, }; # Pass a hashref to a subroutine modify($example_ref); print Dumper("After exiting the subroutine"); print Dumper($example_ref); This will result in: $VAR1 = 'Within the subroutine'; $VAR1 = { 'new_value' => 2, 'old_value' => 1 }; $VAR1 = 'After exiting the subroutine'; $VAR1 = { 'new_value' => 2, 'old_value' => 1 }; Section 3.8: Hashes Hashes can be understood as lookup-tables. You can access its contents by specifiying a key for each of them. Keys must be strings. If they're not, they will be converted to strings. If you give the hash simply a known key, it will serve you its value. # Elements are in (key, value, key, value) sequence my %inhabitants_of = ("London", 8674000, "Paris", 2244000); # You can save some typing and gain in clarity by using the "fat comma" # syntactical sugar. It behaves like a comma and quotes what's on the left. my %translations_of_hello = (spanish => 'Hola', german => 'Hallo', swedish => 'Hej'); In the following example, note the brackets and sigil: you access an element of %hash using $hash{key} because the value you want is a scalar. Some consider it good practice to quote the key while others find this style visually noisy. Quoting is only required for keys that could be mistaken for expressions like $hash{'some-key'} my $greeting = $translations_of_hello{'spanish'}; While Perl by default will try to use barewords as strings, + modifier can also be used to indicate to Perl that key should not be interpolated but executed with result of execution being used as a key: my %employee = ( name => 'John Doe', shift => 'night' ); # this example will print 'night' GoalKicker.com – Perl® Notes for Professionals 12 print $employee{shift}; # but this one will execute [shift][1], extracting first element from @_, # and use result as a key print $employee{+shift}; Like with arrays, you can access multiple hash elements at the same time. This is called a hash slice. The resulting value is a list, so use the @ sigil: my @words = @translations_of_hello{'spanish', 'german'}; # ('Hola', 'Hallo') Iterate over the keys of an hash with keys keys will return items in a random order. Combine with sort if you wish. for my $lang (sort keys %translations_of_hello) { say $translations_of_hello{$lang}; } If you do not actually need the keys like in the previous example, values returns the hash's values directly: for my $translation (values %translations_of_hello) { say $translation; } You can also use a while loop with each to iterate over the hash. This way, you will get both the key and the value at the same time, without a separate value lookup. Its use is however discouraged, as each can break in mistifying ways. # DISCOURAGED while (my ($lang, $translation) = each %translations_of_hello) { say $translation; } Access to unset elements returns undef, not an error: my $italian = $translations_of_hello{'italian'}; # undef map and list flattening can be used to create hashes out of arrays. This is a popular way to create a 'set' of values, e.g. to quickly check whether a value is in @elems. This operation usually takes O(n) time (i.e. proportional to the number of elements) but can be done in constant time (O(1)) by turning the list into a hash: @elems = qw(x y x z t); my %set = map { $_ => 1 } @elems; # (x, 1, y, 1, t, 1) my $y_membership = $set{'y'}; # 1 my $w_membership = $set{'w'}; # undef This requires some explanation. The contents of @elems get read into a list, which is processed by map. map accepts a code block that gets called for each value of its input list; the value of the element is available for use in $_. Our code block returns two list elements for each input element: $_, the input element, and 1, just some value. Once you account for list flattening, the outcome is that map { $_ => 1 } @elems turns qw(x y x z t) into (x => 1, y => 1, x => 1, z => 1, t => 1). As those elements get assigned into the hash, odd elements become hash keys and even elements become hash values. When a key is specified multiple times in a list to be assigned to a hash, the last value wins. This effectively discards duplicates. GoalKicker.com – Perl® Notes for Professionals 13 A faster way to turn a list into a hash uses assignment to a hash slice. It uses the x operator to multiply the single element list (1) by the size of @elems, so there is a 1 value for each of the keys in the slice on the left hand side: @elems = qw(x y x z t); my %set; @set{@elems} = (1) x @elems; The following application of hashes also exploits the fact that hashes and lists can often be used interchangeably to implement named function args: sub hash_args { my %args = @_; my %defaults = (foo => 1, bar => 0); my %overrides = (__unsafe => 0); my %settings = (%defaults, %args, %overrides); } # This function can then be called like this: hash_args(foo => 5, bar => 3); # (foo => 5, bar => 3, __unsafe ==> 0) hash_args(); # (foo => 1, bar => 0, __unsafe ==> 0) hash_args(__unsafe => 1) # (foo => 1, bar => 0, __unsafe ==> 0) When used as booleans, hashes are true if they are not empty. GoalKicker.com – Perl® Notes for Professionals 14 Chapter 4: Interpolation in Perl Section 4.1: What is interpolated Perl interpolates variable names: my $name = 'Paul'; print "Hello, $name!\n"; # Hello, Paul! my @char = ('a', 'b', 'c'); print "$char[1]\n"; # b my %map = (a => 125, b => 1080, c => 11); print "$map{a}\n"; # 125 Arrays may be interpolated as a whole, their elements are separated by spaces: my @char = ('a', 'b', 'c'); print "My chars are @char\n"; # My chars are a b c Perl does not interpolate hashes as a whole: my %map = (a => 125, b => 1080, c => 11); print "My map is %map\n"; # My map is %map and function calls (including constants): use constant { PI => '3.1415926' }; print "I like PI\n"; # I like PI print "I like " . PI . "\n"; # I like 3.1415926 Perl interpolates escape sequences starting with \: \t horizontal tab \n newline \r return \f form feed \b backspace \a alarm (bell) \e escape Interpolation of \n depends on the system where program is working: it will produce a newline character(s) according to the current system conventions. Perl does not interpolate \v, which means vertical tab in C and other languages. Character may be addressed using their codes: \x{1d11e} ???? by hexadecimal code \o{350436} ???? by octal code \N{U+1d11e} ???? by Unicode code point or Unicode names: GoalKicker.com – Perl® Notes for Professionals 15 \N{MUSICAL SYMBOL G CLEF} Character with codes from 0x00 to 0xFF in the native encoding may be addressed in a shorter form: \x0a hexadecimal \012 octal Control character may be addressed using special escape sequences: \c@ chr(0) \ca chr(1) \cb chr(2) ... \cz chr(26) \c[ chr(27) \c\ chr(28) # Cannot be used at the end of a string # since backslash will interpolate the terminating quote \c] chr(29) \c^ chr(30) \c_ chr(31) \c? chr(127) Uppercase letters have the same meaning: "\cA" == "\ca". Interpretation of all escape sequences except for \N{...} may depend on the platform since they use platform and encoding-dependent codes. Section 4.2: Basic interpolation Interpolation means that Perl interpreter will substitute the values of variables for their name and some symbols (which are impossible or difficult to type in directly) for special sequences of characters (it is also known as escaping). The most important distinction is between single and double quotes: double quotes interpolate the enclosed string, but single quotes do not. my $name = 'Paul'; my $age = 64; print "My name is $name.\nI am $age.\n"; # My name is Paul. # I am 64. But: print 'My name is $name.\nI am $age.\n'; # My name is $name.\nI am $age.\n You can use q{} (with any delimiter) instead of single quotes and qq{} instead of double quotes. For example, q{I'm 64} allows to use an apostrophe within a non-interpolated string (otherwise it would terminate the string). Statements: print qq{$name said: "I'm $age".}; # Paul said: "I'm 64". print "$name said: \"I'm $age\"." # Paul said: "I'm 64". do the same thing, but in the first one you do not need to escape double quotes within the string. If your variable name clashes with surrounding text, you can use the syntax ${var} to disambiguate: my $decade = 80; GoalKicker.com – Perl® Notes for Professionals 16 print "I like ${decade}s music!" # I like 80s music! GoalKicker.com – Perl® Notes for Professionals 17 Chapter 5: True and false Section 5.1: List of true and false values use feature qw( say ); # Numbers are true if they're not equal to 0. say 0 ? 'true' : 'false'; # false say 1 ? 'true' : 'false'; # true say 2 ? 'true' : 'false'; # true say -1 ? 'true' : 'false'; # true say 1-1 ? 'true' : 'false'; # false say 0e7 ? 'true' : 'false'; # false say -0.00 ? 'true' : 'false'; # false # Strings are true if they're not empty. say 'a' ? 'true' : 'false'; # true say 'false' ? 'true' : 'false'; # true say '' ? 'true' : 'false'; # false # Even if a string would be treated as 0 in numeric context, it's true if nonempty. # The only exception is the string "0", which is false. # To force numeric context add 0 to the string say '0' ? 'true' : 'false'; # false say '0.0' ? 'true' : 'false'; # true say '0e0' ? 'true' : 'false'; # true say '0 but true' ? 'true' : 'false'; # true say '0 whargarbl' ? 'true' : 'false'; # true say 0+'0 argarbl' ? 'true' : 'false'; # false # Things that become numbers in scalar context are treated as numbers. my @c = (); my @d = (0); say @c ? 'true' : 'false'; # false say @d ? 'true' : 'false'; # true # Anything undefined is false. say undef ? 'true' : 'false'; # false # References are always true, even if they point at something false my @c = (); my $d = 0; say \@c ? 'true' : 'false'; # true say \$d ? 'true' : 'false'; # true say \0 ? 'true' : 'false'; # true say \'' ? 'true' : 'false'; # true GoalKicker.com – Perl® Notes for Professionals 18 Chapter 6: Dates and Time Section 6.1: Date formatting Time::Piece is available in perl 5 after version 10 use Time::Piece; my $date = localtime->strftime('%m/%d/%Y'); print $date; Output 07/26/2016 Section 6.2: Create new DateTime Install DateTime on your PC and then use it in perl script: use DateTime; Create new current datetime $dt = DateTime->now( time_zone => 'Asia/Ho_Chi_Minh'); Then you can access elements's values of date and time: $year = $dt->year; $month = $dt->month; $day = $dt->day; $hour = $dt->hour; $minute = $dt->minute; $second = $dt->second; To get only time: my $time = $dt->hms; #return time with format hh:mm:ss To get only date: my $date = $dt->ymd; #return date with format yyyy-mm-dd Section 6.3: Working with elements of datetime Set single element: $dt->set( year => 2016 ); Set many elements: $dt->set( year => 2016, 'month' => 8); Add duration to datetime $dt->add( hour => 1, month => 2) Datetime subtraction: GoalKicker.com – Perl® Notes for Professionals 19 my $dt1 = DateTime->new( year => 2016, month => 8, day => 20, ); my $dt2 = DateTime->new( year => 2016, month => 8, day => 24, ); my $duration = $dt2->subtract_datetime($dt1); print $duration->days You will get the result is 4 days Section 6.4: Calculate code execution time use Time::HiRes qw( time ); my $start = time(); #Code for which execution time is calculated sleep(1.2); my $end = time(); printf("Execution Time: %0.02f s\n", $end - $start); This will print execution time of Code in seconds GoalKicker.com – Perl® Notes for Professionals 20 Chapter 7: Control Statements Section 7.1: Conditionals Perl supports many kinds of conditional statements (statements that are based on boolean results). The most common conditional statements are if-else, unless, and ternary statements. given statements are introduced as a switch-like construct from C-derived languages and are available in versions Perl 5.10 and above. If-Else Statements The basic structure of an if-statement is like this: if (EXPR) BLOCK if (EXPR) BLOCK else BLOCK if (EXPR) BLOCK elsif (EXPR) BLOCK ... if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK For simple if-statements, the if can precede or succeed the code to be executed. $number = 7; if ($number > 4) { print "$number is greater than four!"; } # Can also be written this way print "$number is greater than four!" if $number > 4; Section 7.2: Loops Perl supports many kinds of loop constructs: for/foreach, while/do-while, and until. @numbers = 1..42; for (my $i=0; $i <= $#numbers; $i++) { print "$numbers[$i]\n"; } #Can also be written as foreach my $num (@numbers) { print "$num\n"; } The while loop evaluates the conditional before executing the associated block. So, sometimes the block is never executed. For example, the following code would never be executed if the filehandle $fh was the filehandle for an empty file, or if was already exhausted before the conditional. while (my $line = readline $fh) { say $line; } The do/while and do/until loops, on the other hand, evaluate the conditional after each time the block is executed. So, a do/while or a do/until loop is always executed at least once. my $greeting_count = 0; do { say "Hello"; $greeting_count++; } until ( $greeting_count > 1) GoalKicker.com – Perl® Notes for Professionals 21 # Hello # Hello GoalKicker.com – Perl® Notes for Professionals 22 Chapter 8: Subroutines Section 8.1: Creating subroutines Subroutines are created by using the keyword sub followed by an identifier and a code block enclosed in braces. You can access the arguments by using the special variable @_, which contains all arguments as an array. sub function_name { my ($arg1, $arg2, @more_args) = @_; # ... } Since the function shift defaults to shifting @_ when used inside a subroutine, it's a common pattern to extract the arguments sequentially into local variables at the beginning of a subroutine: sub function_name { my $arg1 = shift; my $arg2 = shift; my @more_args = @_; # ... } # emulate named parameters (instead of positional) sub function_name { my %args = (arg1 => 'default', @_); my $arg1 = delete $args{arg1}; my $arg2 = delete $args{arg2}; # ... } sub { my $arg1 = shift; # ... }->($arg); Version ≥ 5.20.0 Alternatively, the experimental feature "signatures" can be used to unpack parameters, which are passed by value (not by reference). use feature "signatures"; sub function_name($arg1, $arg2, @more_args) { # ... } Default values can be used for the parameters. use feature "signatures"; sub function_name($arg1=1, $arg2=2) { # ... } You can use any expression to give a default value to a parameter – including other parameters. sub function_name($arg1=1, $arg2=$arg1+1) { GoalKicker.com – Perl® Notes for Professionals 23 # ... } Note that you can't reference parameters which are defined after the current parameter – hence the following code doesn't work quite as expected. sub function_name($arg1=$arg2, $arg2=1) { print $arg1; # => print $arg2; # => 1 } Section 8.2: Subroutines Subroutines hold code. Unless specified otherwise, they are globally defined. # Functions do not (have to) specify their argument list sub returns_one { # Functions return the value of the last expression by default # The return keyword here is unnecessary, but helps readability. return 1; } # Its arguments are available in @_, however sub sum { my $ret = 0; for my $value (@_) { $ret += $value } return $ret; } # Perl makes an effort to make parens around argument list optional say sum 1..3; # 6 # If you treat functions as variables, the & sigil is mandatory. say defined ∑ # 1 Some builtins such as print or say are keywords, not functions, so e.g. &say is undefined. It also does mean that you can define them, but you will have to specify the package name to actually call them # This defines the function under the default package, 'main' sub say { # This is instead the say keyword say "I say, @_"; } # ...so you can call it like this: main::say('wow'); # I say, wow. Version ≥ 5.18.0 Since Perl 5.18, you can also have non-global functions: use feature 'lexical_subs'; my $value; { # Nasty code ahead my sub prod { my $ret = 1; $ret *= $_ for @_; GoalKicker.com – Perl® Notes for Professionals 24 $ret; } $value = prod 1..6; # 720 say defined ∏ # 1 } say defined ∏ # 0 Version ≥ 5.20.0 Since 5.20, you can also have named parameters. use feature 'signatures'; sub greet($name) { say "Hello, $name"; } This should not be confused with prototypes, a facility Perl has to let you define functions that behave like built-ins. Function prototypes must be visible at compile time and its effects can be ignored by specifying the & sigil. Prototypes are generally considered to be an advanced feature that is best used with great care. # This prototype makes it a compilation error to call this function with anything # that isn't an array. Additionally, arrays are automatically turned into arrayrefs sub receives_arrayrefs(\@\@) { my $x = shift; my $y = shift; } my @a = (1..3); my @b = (1..4); receives_arrayrefs(@a, @b); # okay, $x = \@a, $y = \@b, @_ = (); receives_arrayrefs(\@a, \@b); # compilation error, "Type … must be array …" BEGIN { receives_arrayrefs(\@a, \@b); } # Specify the sigil to ignore the prototypes. &receives_arrayrefs(\@a, \@b); # okay, $x = \@a, $y = \@b, @_ = (); &receives_arrayrefs(@a, @b); # ok, but $x = 1, $y = 2, @_ = (3,1,2,3,4); Section 8.3: Subroutine arguments are passed by reference (except those in signatures) Subroutine arguments in Perl are passed by reference, unless they are in the signature. This means that the members of the @_ array inside the sub are just aliases to the actual arguments. In the following example, $text in the main program is left modified after the subroutine call because $_[0] inside the sub is actually just a different name for the same variable. The second invocation throws an error because a string literal is not a variable and therefore can't be modified. use feature 'say'; sub edit { $_[0] =~ s/world/sub/; } my $text = "Hello, world!"; edit($text); say $text; # Hello, sub! edit("Hello, world!"); # Error: Modification of a read-only value attempted To avoid clobbering your caller's variables it is therefore important to copy @_ to locally scoped variables (my ...) as GoalKicker.com – Perl® Notes for Professionals 25 described under "Creating subroutines". GoalKicker.com – Perl® Notes for Professionals 26 Chapter 9: Debug Output Section 9.1: Dumping with Style Sometimes Data::Dumper is not enough. Got a Moose object you want to inspect? Huge numbers of the same structure? Want stuff sorted? Colored? Data::Printer is your friend. use Data::Printer; p $data_structure; Data::Printer writes to STDERR, like warn. That makes it easier to find the output. By default, it sorts hash keys and looks at objects. use Data::Printer; use LWP::UserAgent; my $ua = LWP::UserAgent->new; p $ua; It will look at all the methods of the object, and also list the internals. LWP::UserAgent { Parents LWP::MemberMixin public methods (45) : add_handler, agent, clone, conn_cache, cookie_jar, credentials, default_header, default_headers, delete, env_proxy, from, get, get_basic_credentials, get_my_handler, handlers, head, is_online, is_protocol_supported, local_address, max_redirect, max_size, mirror, new, no_proxy, parse_head, post, prepare_request, progress, protocols_allowed, protocols_forbidden, proxy, put, redirect_ok, remove_handler, request, requests_redirectable, run_handlers, send_request, set_my_handler, show_progress, simple_request, ssl_opts, timeout, use_alarm, use_eval private methods (4) : _agent, _need_proxy, _new_response, _process_colonic_headers internals: { def_headers HTTP::Headers, handlers { response_header HTTP::Config }, local_address undef, max_redirect 7, max_size undef, no_proxy [], protocols_allowed undef, protocols_forbidden undef, proxy {}, requests_redirectable [ [0] "GET", [1] "HEAD" ], show_progress undef, ssl_opts { verify_hostname 1 }, timeout 180, use_eval 1 GoalKicker.com – Perl® Notes for Professionals 27 } } You can configure it further, so it serializes certain objects in a certain way, or to include objects up to an arbitrary depth. The full configuration is available in the documentation. Unfortunately Data::Printer does not ship with Perl, so you need to install it from CPAN or through your package management system. Section 9.2: Dumping data-structures use Data::Dumper; my $data_structure = { foo => 'bar' }; print Dumper $data_structure; Using Data::Dumper is an easy way to look at data structures or variable content at run time. It ships with Perl and you can load it easily. The Dumper function returns the data structure serialized in a way that looks like Perl code. $VAR1 = { 'foo' => 'bar', } That makes it very useful to quickly look at some values in your code. It's one of the most handy tools you have in your arsenal. Read the full documentation on metacpan. Section 9.3: Data::Show The function show is automatically exported when use Data::Show; is executed. This function takes a variable as its sole argument and it outputs: 1. the name of the variable 2. the contents of that variable (in a readable format) 3. the line of the file that show is run from 4. the file show is run from Assuming that the following is code from the file example.pl: use strict; use warnings; use Data::Show; my @array = (1, 2, 3, 4, 5, 6, 7, 8, 9, 10); my %hash = ( foo => 1, bar => { baz => 10, qux => 20 } ); my $href = \%hash; show @array; show %hash; show $href; perl example.pl gives the following output: ======( @array )=======================[ 'example.pl', line 11 ]====== GoalKicker.com – Perl® Notes for Professionals 28 [1 .. 10] ======( %hash )========================[ 'example.pl', line 12 ]====== { bar => { baz => 10, qux => 20 }, foo => 1 } ======( $href )========================[ 'example.pl', line 13 ]====== { bar => { baz => 10, qux => 20 }, foo => 1 } See the documentation for Data::Show. Section 9.4: Dumping array list my @data_array = (123, 456, 789, 'poi', 'uyt', "rew", "qas"); print Dumper @data_array; Using Data::Dumper gives an easy access to fetch list values. The Dumper returns the list values serialized in a way that looks like Perl code. Output: $VAR1 = 123; $VAR2 = 456; $VAR3 = 789; $VAR4 = 'poi'; $VAR5 = 'uyt'; $VAR6 = 'rew'; $VAR7 = 'qas'; As suggested by user @dgw When dumping arrays or hashes it is better to use an array reference or a hash reference, those will be shown better fitting to the input. $ref_data = [23,45,67,'mnb','vcx']; print Dumper $ref_data; Output: $VAR1 = [ 23, 45, 67, 'mnb', 'vcx' ]; You can also reference the array when printing. my @data_array = (23,45,67,'mnb','vcx'); print Dumper \@data_array; Output: $VAR1 = [ 23, GoalKicker.com – Perl® Notes for Professionals 29 45, 67, 'mnb', 'vcx' ]; GoalKicker.com – Perl® Notes for Professionals 30 Chapter 10: Lists Section 10.1: Array as list The array is one of Perl's basic variable types. It contains a list, which is an ordered sequence of zero or more scalars. The array is the variable holding (and providing access to) the list data, as is documented in perldata. You can assign a list to an array: my @foo = ( 4, 5, 6 ); You can use an array wherever a list is expected: join '-', ( 4, 5, 6 ); join '-', @foo; Some operators only work with arrays since they mutate the list an array contains: shift @array; unshift @array, ( 1, 2, 3 ); pop @array; push @array, ( 7, 8, 9 ); Section 10.2: Assigning a list to a hash Lists can also be assigned to hash variables. When creating a list that will be assigned to a hash variable, it is recommended to use the fat comma => between keys and values to show their relationship: my %hash = ( foo => 42, bar => 43, baz => 44 ); The => is really only a special comma that automatically quotes the operand to its left. So, you could use normal commas, but the relationship is not as clear: my %hash = ( 'foo', 42, 'bar', 43, 'baz', 44 ); You can also use quoted strings for the left hand operand of the fat comma =>, which is especially useful for keys containing spaces. my %hash = ( 'foo bar' => 42, 'baz qux' => 43 ); For details see Comma operator at perldoc perlop. Section 10.3: Lists can be passed into subroutines As to pass list into a subroutine, you specify the subroutine's name and then supply the list to it: test_subroutine( 'item1', 'item2' ); test_subroutine 'item1', 'item2'; # same Internally Perl makes aliases to those arguments and put them into the array @_ which is available within the subroutine: @_ = ( 'item1', 'item2' ); # Done internally by perl GoalKicker.com – Perl® Notes for Professionals 31 You access subroutine arguments like this: sub test_subroutine { print $_[0]; # item1 print $_[1]; # item2 } Aliasing gives you the ability to change the original value of argument passed to subroutine: sub test_subroutine { $_[0] += 2; } my $x = 7; test_subroutine( $x ); print $x; # 9 To prevent inadvertent changes of original values passed into your subroutine, you should copy them: sub test_subroutine { my( $copy_arg1, $copy_arg2 ) = @_; $copy_arg1 += 2; } my $x = 7; test_subroutine $x; # in this case $copy_arg2 will have `undef` value print $x; # 7 To test how many arguments were passed into the subroutine, check the size of @_ sub test_subroutine { print scalar @_, ' argument(s) passed into subroutine'; } If you pass array arguments into a subroutine they all will be flattened: my @x = ( 1, 2, 3 ); my @y = qw/ a b c /; # ( 'a', 'b', 'c' ) test_some_subroutine @x, 'hi', @y; # 7 argument(s) passed into subroutine # @_ = ( 1, 2, 3, 'hi', 'a', 'b', 'c' ) # Done internally for this call If your test_some_subroutine contains the statement $_[4] = 'd', for the above call it will cause $y[0] to have value d afterwards: print "@y"; # d b c Section 10.4: Return list from subroutine You can, of course, return lists from subs: sub foo { my @list1 = ( 1, 2, 3 ); my @list2 = ( 4, 5 ); return ( @list1, @list2 ); } GoalKicker.com – Perl® Notes for Professionals 32 my @list = foo(); print @list; # 12345 But it is not the recommended way to do that unless you know what you are doing. While this is OK when the result is in LIST context, in SCALAR context things are unclear. Let's take a look at the next line: print scalar foo(); # 2 Why 2? What is going on? 1. Because foo() evaluated in SCALAR context, this list ( @list1, @list2 ) also evaluated in SCALAR context 2. In SCALAR context, LIST returns its last element. Here it is @list2 3. Again in SCALAR context, array @list2 returns the number of its elements. Here it is 2. In most cases the right strategy will return references to data structures. So in our case we should do the following instead: return ( \@list1, \@list2 ); Then the caller does something like this to receive the two returned arrayrefs: my ($list1, $list2) = foo(...); Section 10.5: Hash as list In list context hash is flattened. my @bar = ( %hash, %hash ); The array @bar is initialized by list of two %hash hashes both %hash are flattened new list is created from flattened items @bar array is initialized by that list It is guaranteed that key-value pairs goes together. Keys are always even indexed, values - odd. It is not guaranteed that key-value pairs are always flattened in same order: my %hash = ( a => 1, b => 2 ); print %hash; # Maybe 'a1b2' or 'b2a1' Section 10.6: Using arrayref to pass array to sub The arrayref for @foo is \@foo. This is handy if you need to pass an array and other things to a subroutine. Passing @foo is like passing multiple scalars. But passing \@foo is a single scalar. Inside the subroutine: xyz(\@foo, 123); ... sub xyz { my ($arr, $etc) = @_; print $arr->[0]; # using the first item in $arr. It is like $foo[0] GoalKicker.com – Perl® Notes for Professionals 33 Chapter 11: Sorting For sorting lists of things, Perl has only a single function, unsurprisingly called sort. It is flexible enough to sort all kinds of items: numbers, strings in any number of encodings, nested data structures or objects. However, due to its flexibility, there are quite a few tricks and idioms to be learned for its use. Section 11.1: Basic Lexical Sort @sorted = sort @list; @sorted = sort { $a cmp $b } @list; sub compare { $a cmp $b } @sorted = sort compare @list; The three examples above do exactly the same thing. If you don't supply any comparator function or block, sort assumes you want the list on its right sorted lexically. This is usually the form you want if you just need your data in some predictable order and don't care about linguistic correctness. sort passes pairs of items in @list to the comparator function, which tells sort which item is larger. The cmp operator does this for strings while <=> does the same thing for numbers. The comparator is called quite often, on average n*log(n) times with n being the number of elements to be sorted, so it's important it be fast. This is the reason sort uses predefined package global variables ($a and $b) to pass the elements to be compared to the block or function, instead of proper function parameters. If you use locale, cmp takes locale specific collation order into account, e.g. it will sort Å like A under a Danish locale but after Z under an English or German one. However, it doesn't take the more complex Unicode sorting rules into account nor does it offer any control over the order—for example phone books are often sorted differently from dictionaries. For those cases, the Unicode::Collate and particularly Unicode::Collate::Locale modules are recommended. Section 11.2: The Schwartzian Transform This is probably the most famous example of a sort optimization making use of Perl's functional programming facilities, to be used where the sort order of items depend on an expensive function. # What you would usually do @sorted = sort { slow($a) <=> slow($b) } @list; # What you do to make it faster @sorted = map { $_->[0] } sort { $a->[1] <=> $b->[1] } map { [ $_, slow($_) ] } @list; The trouble with the first example is that the comparator is called very often and keeps recalculating values using a slow function over and over. A typical example would be sorting file names by their file size: use File::stat; @sorted = sort { stat($a)->size <=> stat($b)->size } glob "*"; This works, but at best it incurs the overhead of two system calls per comparison, at worst it has to go to the disk, twice, for every single comparison, and that disk may be in an overloaded file server on the other side of the planet. GoalKicker.com – Perl® Notes for Professionals 34 Enter Randall Schwartz's trick. The Schwartzian Transform basically shoves @list through three functions, bottom-to-top. The first map turns each entry into a two-element list of the original item and the result of the slow function as a sort key, so at the end of this we have called slow() exactly once for each element. The following sort can then simply access the sort key by looking in the list. As we don't care about the sort keys but only need the original elements in sorted order, the final map throws away the two-element lists from the already-sorted list it receives from @sort and returns a list of only their first members. Section 11.3: Case Insensitive Sort The traditional technique to make sort ignore case is to pass strings to lc or uc for comparison: @sorted = sort { lc($a) cmp lc($b) } @list; This works on all versions of Perl 5 and is completely sufficient for English; it doesn't matter whether you use uc or lc. However, it presents a problem for languages like Greek or Turkish where there is no 1:1 correspondence between upper- and lowercase letters so you get different results depending on whether you use uc or lc. Therefore, Perl 5.16 and higher have a case folding function called fc that avoids this problem, so modern multi lingual sorting should use this: @sorted = sort { fc($a) cmp fc($b) } @list; Section 11.4: Numeric Sort @sorted = sort { $a <=> $b } @list; Comparing $a and $b with the <=> operator ensures they are compared numerically and not textually as per default. Section 11.5: Reverse Sort @sorted = sort { $b <=> $a } @list; @sorted = reverse sort { $a <=> $b } @list; Sorting items in descending order can simply be achieved by swapping $a and $b in the comparator block. However, some people prefer the clarity of a separate reverse even though it is slightly slower. GoalKicker.com – Perl® Notes for Professionals 35 Chapter 12: File I/O (reading and writing files) Mode Explaination > Write (trunc). Will overwrite existing files. Creates a new file if no file was found >>Write (append). Will not overwrite files but append new content at the end of it. Will also create a file if used for opening a non existing file. < Read. Opens the file in read only mode. +< Read / Write. Will not create or truncate the file. +> Read / Write (trunc). Will create and truncate the file. +>> Read / Write (append). Will create but not truncate the file. Section 12.1: Opening A FileHandle for Reading Opening Generic ASCII Text Files Version ≥ 5.6.0 open my $filehandle, '<', $name_of_file or die "Can't open $name_of_file, $!"; This is the basic idiom for "default" File IO and makes $filehandle a readable input stream of bytes, filtered by a default system-specific decoder, which can be locally set with the open pragma Perl itself does not handle errors in file opening, so you have to handle those yourself by checking the exit condition of open. $! is populated with the error message that caused open to fail. On Windows, the default decoder is a "CRLF" filter, which maps any "\r\n" sequences in the input to "\n" Opening Binary Files Version ≥ 5.8.0 open my $filehandle, '<:raw', 'path/to/file' or die "Can't open $name_of_file, $!"; This specifies that Perl should not perform a CRLF translation on Windows. Opening UTF8 Text Files Version ≥ 5.8.0 open my $filehandle, '<:raw:encoding(utf-8)', 'path/to/file' or die "Can't open $name_of_file, $!"; This specifies that Perl should both avoid CRLF translation, and then decode the resulting bytes into strings of characters ( internally implemented as arrays of integers which can exceed 255 ), instead of strings of bytes Section 12.2: Reading from a file my $filename = '/path/to/file'; open my $fh, '<', $filename or die "Failed to open file: $filename"; # You can then either read the file one line at a time... while(chomp(my $line = <$fh>)) { print $line . "\n"; } # ...or read whole file into an array in one go GoalKicker.com – Perl® Notes for Professionals 36 chomp(my @fileArray = <$fh>); If you know that your input file is UTF-8, you can specify the encoding: open my $fh, '<:encoding(utf8)', $filename or die "Failed to open file: $filename"; After finished reading from the file, the filehandle should be closed: close $fh or warn "close failed: $!"; See also: Reading a file into a variable Another and faster way to read a file is to use File::Slurper Module. This is useful if you work with many files. use File::Slurper; my $file = read_text("path/to/file"); # utf8 without CRLF transforms by default print $file; #Contains the file body See also: [Reading a file with slurp] Section 12.3: Write to a file This code opens a file for writing. Returns an error if the file couldn't be opened. Also closes the file at the end. #!/usr/bin/perl use strict; use warnings; use open qw( :encoding(UTF-8) :std ); # Make UTF-8 default encoding # Open "output.txt" for writing (">") and from now on, refer to it as the variable $fh. open(my $fh, ">", "output.txt") # In case the action failed, print error message and quit. or die "Can't open > output.txt: $!"; Now we have an open file ready for writing which we access through $fh (this variable is called a filehandle). Next we can direct output to that file using the print operator: # Print "Hello" to $fh ("output.txt"). print $fh "Hello"; # Don't forget to close the file once we're done! close $fh or warn "Close failed: $!"; The open operator has a scalar variable ($fh in this case) as its first parameter. Since it is defined in the open operator it is treated as a filehandle. Second parameter ">" (greater than) defines that the file is opened for writing. The last parameter is the path of the file to write the data to. To write the data into the file, the print operator is used along with the filehandle. Notice that in the print operator there is no comma between the filehandle and the statement itself, just whitespace. Section 12.4: "use autodie" and you won't need to check file open/close failures autodie allows you to work with files without having to explicitly check for open/close failures. Since Perl 5.10.1, the autodie pragma has been available in core Perl. When used, Perl will automatically check for GoalKicker.com – Perl® Notes for Professionals 37 errors when opening and closing files. Here is an example in which all of the lines of one file are read and then written to the end of a log file. use 5.010; # 5.010 and later enable "say", which prints arguments, then a newline use strict; # require declaring variables (avoid silent errors due to typos) use warnings; # enable helpful syntax-related warnings use open qw( :encoding(UTF-8) :std ); # Make UTF-8 default encoding use autodie; # Automatically handle errors in opening and closing files open(my $fh_in, '<', "input.txt"); # check for failure is automatic # open a file for appending (i.e. using ">>") open( my $fh_log, '>>', "output.log"); # check for failure is automatic while (my $line = readline $fh_in) # also works: while (my $line = <$fh_in>) { # remove newline chomp $line; # write to log file say $fh_log $line or die "failed to print '$line'"; # autodie doesn't check print } # Close the file handles (check for failure is automatic) close $fh_in; close $fh_log; By the way, you should technically always check print statements. Many people don't, but perl (the Perl interpreter) doesn't do this automatically and neither does autodie. Section 12.5: Rewind a filehandle Sometimes it is needful to backtrack after reading. # identify current position in file, in case the first line isn't a comment my $current_pos = tell; while (my $line = readline $fh) { if ($line =~ /$START_OF_COMMENT_LINE/) { push @names, get_name_from_comment($line); } else { last; # break out of the while loop } $current_pos = tell; # keep track of current position, in case we need to rewind the next line read } # Step back a line so that it can be processed later as the first data line seek $fh, $current_pos, 0; Section 12.6: Reading and Writing gzip compressed files Writing a gzipped file To write a gzipped file, use the module IO::Compress::Gzip and create a filehandle by creating a new instance of GoalKicker.com – Perl® Notes for Professionals 38 IO::Compress::Gzip for the desired output file: use strict; use warnings; use open qw( :encoding(UTF-8) :std ); # Make UTF-8 default encoding use IO::Compress::Gzip; my $fh_out = IO::Compress::Gzip->new("hello.txt.gz"); print $fh_out "Hello World!\n"; close $fh_out; use IO::Compress::Gzip; Reading from a gzipped file To read from a gzipped file, use the module IO::Uncompress::Gunzip and then create a filehandle by creating a new instance of IO::Uncompress::Gunzip for the input file: #!/bin/env perl use strict; use warnings; use open qw( :encoding(UTF-8) :std ); # Make UTF-8 default encoding use IO::Uncompress::Gunzip; my $fh_in = IO::Uncompress::Gunzip->new("hello.txt.gz"); my $line = readline $fh_in; print $line; Section 12.7: Setting the default Encoding for IO # encode/decode UTF-8 for files and standard input/output use open qw( :encoding(UTF-8) :std ); This pragma changes the default mode of reading and writing text ( files, standard input, standard output, and standard error ) to UTF-8, which is typically what you want when writing new applications. ASCII is a subset of UTF-8, so this is not expected to cause any problems with legacy ASCII files and will help protect you the accidental file corruption that can happen when treating UTF-8 files as ASCII. However, it is important that you know what the encoding of your files is that you are dealing with and handle them accordingly. (Reasons that we should not ignore Unicode.) For more in depth treatment of Unicode, please see the Perl Unicode topic. GoalKicker.com – Perl® Notes for Professionals 39 Chapter 13: Reading a file's content into a variable Section 13.1: Path::Tiny Using the idiom from The Manual Way several times in a script soon gets tedious so you might want to try a module. use Path::Tiny; my $contents = path($filename)->slurp; You can pass a binmode option if you need control over file encodings, line endings etc. - see man perlio: my $contents = path($filename)->slurp( {binmode => ":encoding(UTF-8)"} ); Path::Tiny also has a lot of other functions for dealing with files so it may be a good choice. Section 13.2: The manual way open my $fh, '<', $filename or die "Could not open $filename for reading: $!"; my $contents = do { local $/; <$fh> }; After opening the file (read man perlio if you want to read specific file encodings instead of raw bytes), the trick is in the do block: <$fh>, the file handle in a diamond operator, returns a single record from the file. The "input record separator" variable $/ specifies what a "record" is—by default it is set to a newline character so "a record" means "a single line". As $/ is a global variable, local does two things: it creates a temporary local copy of $/ that will vanish at the end of the block, and gives it the (non-)value undef (the "value" which Perl gives to uninitialized variables). When the input record separator has that (non-)value, the diamond operator will return the entire file. (It considers the entire file to be a single line.) Using do, you can even get around manually opening a file. For repeated reading of files, sub readfile { do { local(@ARGV,$/) = $_[0]; <> } } my $content = readfile($filename); can be used. Here, another global variable(@ARGV) is localized to simulate the same process used when starting a perl script with parameters. $/ is still undef, since the array in front of it "eats" all incoming arguments. Next, the diamond operator <> again delivers one record defined by $/ (the whole file) and returns from the do block, which in turn return from the sub. The sub has no explicit error handling, which is bad practice! If an error occurs while reading the file, you will receive undef as return value, as opposed to an empty string from an empty file. Another disadvantage of the last code is the fact that you cannot use PerlIO for different file encodings—you always get raw bytes. Section 13.3: File::Slurp Don't use it. Although it has been around for a long time and is still the module most programmers will suggest, it is broken and not likely to be fixed. GoalKicker.com – Perl® Notes for Professionals 40 Section 13.4: File::Slurper This is a minimalist module that only slurps files into variables, nothing else. use File::Slurper 'read_text'; my $contents = read_text($filename); read_text() takes two optional parameters to specify the file encoding and whether line endings should be translated between the unixish LF or DOSish CRLF standards: my $contents = read_text($filename, 'UTF-8', 1); Section 13.5: Slurping a file into an array variable open(my $fh, '<', "/some/path") or die $!; my @ary = <$fh>; When evaluated in list context, the diamond operator returns a list consisting of all the lines in the file (in this case, assigning the result to an array supplies list context). The line terminator is retained, and can be removed by chomping: chomp(@ary); #removes line terminators from all the array elements. Section 13.6: Slurp file in one-liner Input record separator can be specified with -0 switch (zero, not capital O). It takes an octal or hexadecimal number as value. Any value 0400 or above will cause Perl to slurp files, but by convention, the value used for this purpose is 0777. perl -0777 -e 'my $file = <>; print length($file)' input.txt Going further with minimalism, specifying -n switch causes Perl to automatically read each line (in our case — the whole file) into variable $_. perl -0777 -ne 'print length($_)' input.txt GoalKicker.com – Perl® Notes for Professionals 41 Chapter 14: Strings and quoting methods Section 14.1: String Literal Quoting String literals imply no escaping or interpolation ( with the exception of quoting string terminators ) print 'This is a string literal\n'; # emits a literal \ and n to terminal print 'This literal contains a \'postraphe '; # emits the ' but not its preceding \ You can use alternative quoting mechanisms to avoid clashes: print q/This is is a literal \' <-- 2 characters /; # prints both \ and ' print q^This is is a literal \' <-- 2 characters ^; # also Certain chosen quote characters are "balanced" print q{ This is a literal and I contain { parens! } }; # prints inner { } Section 14.2: Double-quoting Double-quoted strings use interpolation and escaping – unlike single-quoted strings. To double-quote a string, use either double quotes " or the qq operator. my $greeting = "Hello!\n"; print $greeting; # => Hello! (followed by a linefeed) my $bush = "They misunderestimated me." print qq/As Bush once said: "$bush"\n/; # => As Bush once said: "They misunderestimated me." (with linefeed) The qq is useful here, to avoid having to escape the quotation marks. Without it, we would have to write... print "As Bush once said: \"$bush\"\n"; ... which just isn't as nice. Perl doesn't limit you to using a slash / with qq; you can use any (visible) character. use feature 'say'; say qq/You can use slashes.../; say qq{...or braces...}; say qq^...or hats...^; say qq|...or pipes...|; # say qq ...but not whitespace. ; You can also interpolate arrays into strings. use feature 'say'; my @letters = ('a', 'b', 'c'); say "I like these letters: @letters."; GoalKicker.com – Perl® Notes for Professionals 42 # => I like these letters: a b c. By default the values are space-separated – because the special variable $" defaults to a single space. This can, of course, be changed. use feature 'say'; my @letters = ('a', 'b', 'c'); {local $" = ", "; say "@letters"; } # a, b, c If you prefer, you have the option to use English and change $LIST_SEPARATOR instead: use v5.18; # English should be avoided on older Perls use English; my @letters = ('a', 'b', 'c'); { local $LIST_SEPARATOR = "\n"; say "My favourite letters:\n\n@letters" } For anything more complex than this, you should use a loop instead. say "My favourite letters:"; say; for my $letter (@letters) { say " - $letter"; } Interpolation does not work with hashes. use feature 'say'; my %hash = ('a', 'b', 'c', 'd'); say "This doesn't work: %hash" # This doesn't work: %hash Some code abuses interpolation of references – avoid it. use feature 'say'; say "2 + 2 == @{[ 2 + 2 ]}"; # 2 + 2 = 4 (avoid this) say "2 + 2 == ${\( 2 + 2 )}"; # 2 + 2 = 4 (avoid this) The so-called "cart operator" causes perl to dereference @{ ... } the array reference [ ... ] that contains the expression that you want to interpolate, 2 + 2. When you use this trick, Perl builds an anonymous array, then dereferences it and discards it. The ${\( ... )} version is somewhat less wasteful, but it still requires allocating memory and it is even harder to read. Instead, consider writing: say "2 + 2 == " . 2 + 2; my $result = 2 + 2; say "2 + 2 == $result" Section 14.3: Heredocs Large Multi-Line strings are burdensome to write. GoalKicker.com – Perl® Notes for Professionals 43 my $variable = <<'EOF'; this block of text is interpreted literally, no \'quotes matter, they're just text only the trailing left-aligned EOF matters. EOF NB: Make sure you ignore stack-overflows syntax highlighter: It is very wrong. And Interpolated Heredocs work the same way. my $variable = <<"I Want it to End"; this block of text is interpreted. quotes\nare interpreted, and $interpolations get interpolated... but still, left-aligned "I Want it to End" matters. I Want it to End Pending in 5.26.0* is an "Indented Heredoc" Syntax which trims left-padding off for you Version ≥ 5.26.0 my $variable = <<~"MuchNicer"; this block of text is interpreted. quotes\nare interpreted, and $interpolations get interpolated... but still, left-aligned "I Want it to End" matters. MuchNicer Section 14.4: Removing trailing newlines The function chomp will remove one newline character, if present, from each scalar passed to it. chomp will mutate the original string and will return the number of characters removed my $str = "Hello World\n\n"; my $removed = chomp($str); print $str; # "Hello World\n" print $removed; # 1 # chomp again, removing another newline $removed = chomp $str; print $str; # "Hello World" print $removed; # 1 # chomp again, but no newline to remove $removed = chomp $str; print $str; # "Hello World" print $removed; # 0 You can also chomp more than one string at once: my @strs = ("Hello\n", "World!\n\n"); # one newline in first string, two in second my $removed = chomp(@strs); # @strs is now ("Hello", "World!\n") print $removed; # 2 $removed = chomp(@strs); # @strs is now ("Hello", "World!") print $removed; # 1 $removed = chomp(@strs); # @strs is still ("Hello", "World!") GoalKicker.com – Perl® Notes for Professionals 44 print $removed; # 0 But usually, no one worries about how many newlines were removed, so chomp is usually seen in void context, and usually due to having read lines from a file: while (my $line = readline $fh) { chomp $line; # now do something with $line } my @lines = readline $fh2; chomp (@lines); # remove newline from end of each line GoalKicker.com – Perl® Notes for Professionals 45 Chapter 15: Split a string on unquoted separators Section 15.1: parse_line() Using parse_line() of Text::ParseWords: use 5.010; use Text::ParseWords; my $line = q{"a quoted, comma", word1, word2}; my @parsed = parse_line(',', 1, $line); say for @parsed; Output: "a quoted, comma" word1 word2 Section 15.2: Text::CSV or Text::CSV_XS use Text::CSV; # Can use Text::CSV which will switch to _XS if installed $sep_char = ","; my $csv = Text::CSV->new({sep_char => $sep_char}); my $line = q{"a quoted, comma", word1, word2}; $csv->parse($line); my @fields = $csv->fields(); print join("\n", @fields)."\n"; Output: a quoted, comma word1 word2 NOTES By default, Text::CSV does not strip whitespace around separator character, the way Text::ParseWords does. However, adding allow_whitespace=>1 to constructor attributes achieves that effect. my $csv = Text::CSV_XS->new({sep_char => $sep_char, allow_whitespace=>1}); Output: a quoted, comma word1 word2 The library supports escaping special characters (quotes, separators) The library supports configurable separator character, quote character, and escape character Documentatoin: http://search.cpan.org/perldoc/Text::CSV GoalKicker.com – Perl® Notes for Professionals 46 Chapter 16: Object-oriented Perl Section 16.1: Defining classes in modern Perl Although available, defining a class from scratch is not recommended in modern Perl. Use one of helper OO systems which provide more features and convenience. Among these systems are: Moose - inspired by Perl 6 OO design Class::Accessor - a lightweight alternative to Moose Class::Tiny - truly minimal class builder Moose package Foo; use Moose; has bar => (is => 'ro'); # a read-only property has baz => (is => 'rw', isa => 'Bool'); # a read-write boolean property sub qux { my $self = shift; my $barIsBaz = $self->bar eq 'baz'; # property getter $self->baz($barIsBaz); # property setter } Class::Accessor (Moose syntax) package Foo; use Class::Accessor 'antlers'; has bar => (is => 'ro'); # a read-only property has baz => (is => 'rw', isa => 'Bool'); # a read-write property (only 'is' supported, the type is ignored) Class::Accessor (native syntax) package Foo; use base qw(Class::Accessor); Foo->mk_accessors(qw(bar baz)); # some read-write properties Foo->mk_accessors(qw(qux)); # a read-only property Class::Tiny package Foo; use Class::Tiny qw(bar baz); # just props Section 16.2: Creating Objects Unlike many other languages, Perl does not have constructors that allocate memory for your objects. Instead, one should write a class method that both create a data structure and populate it with data (you may know it as the Factory Method design pattern). GoalKicker.com – Perl® Notes for Professionals 47 package Point; use strict; sub new { my ($class, $x, $y) = @_; my $self = { x => $x, y => $y }; # store object data in a hash bless $self, $class; # bind the hash to the class return $self; } This method can be used as follows: my $point = Point->new(1, 2.5); Whenever the arrow operator -> is used with methods, its left operand is prepended to the given argument list. So, @_ in new will contain values ('Point', 1, 2.5). There is nothing special in the name new. You can call the factory methods as you prefer. There is nothing special in hashes. You could do the same in the following way: package Point; use strict; sub new { my ($class, @coord) = @_; my $self = \@coord; bless $self, $class; return $self; } In general, any reference may be an object, even a scalar reference. But most often, hashes are the most convenient way to represent object data. Section 16.3: Defining Classes In general, classes in Perl are just packages. They can contain data and methods, as usual packages. package Point; use strict; my $CANVAS_SIZE = [1000, 1000]; sub new { ... } sub polar_coordinates { ... } 1; It is important to note that the variables declared in a package are class variables, not object (instance) variables. Changing of a package-level variable affects all objects of the class. How to store object-specific data, see in "Creating Objects". GoalKicker.com – Perl® Notes for Professionals 48 What makes class packages specific, is the arrow operator ->. It may be used after a bare word: Point->new(...); or after a scalar variable (usually holding a reference): my @polar = $point->polar_coordinates; What is to the left of the arrow is prepended to the given argument list of the method. For example, after call Point->new(1, 2); array @_ in new will contain three arguments: ('Point', 1, 2). Packages representing classes should take this convention into account and expect that all their methods will have one extra argument. Section 16.4: Inheritance and methods resolution To make a class a subclass of another class, use parent pragma: package Point; use strict; ... 1; package Point2D; use strict; use parent qw(Point); ... 1; package Point3D; use strict; use parent qw(Point); ... 1; Perl allows for multiple inheritance: package Point2D; use strict; use parent qw(Point PlanarObject); ... 1; Inheritance is all about resolution which method is to be called in a particular situation. Since pure Perl does not prescribe any rules about the data structure used to store object data, inheritance has nothing to do with that. Consider the following class hierarchy: package GeometryObject; use strict; sub transpose { ...} 1; GoalKicker.com – Perl® Notes for Professionals 49 package Point; use strict; use parent qw(GeometryObject); sub new { ... }; 1; package PlanarObject; use strict; use parent qw(GeometryObject); sub transpose { ... } 1; package Point2D; use strict; use parent qw(Point PlanarObject); sub new { ... } sub polar_coordinates { ... } 1; The method resolution works as follows: 1. The starting point is defined by the left operand of the arrow operator. If it is a bare word: Point2D->new(...); ...or a scalar variable holding a string: my $class = 'Point2D'; $class->new(...); ...then the starting point is the package with the corresponding name (Point2D in both examples). If the left operand is a scalar variable holding a blessed reference: my $point = {...}; bless $point, 'Point2D'; # typically, it is encapsulated into class methods my @coord = $point->polar_coordinates; then the starting point is the class of the reference (again, Point2D). The arrow operator cannot be used to call methods for unblessed references. 2. If the starting point contains the required method, it is simply called. Thus, since Point2D::new exists, Point2D->new(...); will simply call it. GoalKicker.com – Perl® Notes for Professionals 50 3. If the starting point does not contain the required method, depth-first search in the parent classes is performed. In the example above, the search order will be as follows: Point2D Point (first parent of Point2D) GeometryObject (parent of Point) PlanarObject (second parent of Point2D) For example, in the following code: my $point = Point2D->new(...); $point->transpose(...); the method that will be called is GeometryObject::transpose, even though it would be overridden in PlanarObject::transpose. 4. You can set the starting point explicitly. In the previous example, you can explicitly call PlanarObject::transpose like so: my $point = Point2D->new(...); $point->PlanarObject::transpose(...); 5. In a similar manner, SUPER:: performs method search in parent classes of the current class. For example, package Point2D; use strict; use parent qw(Point PlanarObject); sub new { (my $class, $x, $y) = @_; my $self = $class->SUPER::new; ... } 1; will call Point::new in the course of the Point2D::new execution. Section 16.5: Class and Object Methods In Perl, the difference between class (static) and object (instance) methods is not so strong as in some other languages, but it still exists. The left operand of the arrow operator -> becomes the first argument of the method to be called. It may be either a string: # the first argument of new is string 'Point' in both cases Point->new(...); my $class = 'Point'; GoalKicker.com – Perl® Notes for Professionals 51 $class->new(...); or an object reference: # reference contained in $point is the first argument of polar_coordinates my $point = Point->new(...); my @coord = $point->polar_coordinates; Class methods are just the ones that expect their first argument to be a string, and object methods are the ones that expect their first argument to be an object reference. Class methods typically do not do anything with their first argument, which is just a name of the class. Generally, it is only used by Perl itself for method resolution. Therefore, a typical class method can be called for an object as well: my $width = Point->canvas_width; my $point = Point->new(...); my $width = $point->canvas_width; Although this syntax is allowed, it is often misleading, so it is better to avoid it. Object methods receive an object reference as the first argument, so they can address the object data (unlike class methods): package Point; use strict; sub polar_coordinates { my ($point) = @_; my $x = $point->{x}; my $y = $point->{y}; return (sqrt($x * $x + $y * $y), atan2($y, $x)); } 1; The same method can track both cases: when it is called as a class or an object method: sub universal_method { my $self = shift; if (ref $self) { # object logic ... } else { # class logic ... } } Section 16.6: Roles A role in Perl is essentially a set of methods and attributes which injected into a class directly. GoalKicker.com – Perl® Notes for Professionals 52 A role provides a piece of functionality which can be composed into (or applied to) any class (which is said to consume the role). A role cannot be inherited but may be consumed by another role. A role may also require consuming classes to implement some methods instead of implementing the methods itself (just like interfaces in Java or C#). Perl does not have built-in support for roles but there are CPAN classes which provide such support. Moose::Role package Chatty; use Moose::Role; requires 'introduce'; # a method consuming classes must implement sub greet { # a method already implemented in the role print "Hi!\n"; } package Parrot; use Moose; with 'Chatty'; sub introduce { print "I'm Buddy.\n"; } Role::Tiny Use if your OO system does not provide support for roles (e.g. Class::Accessor or Class::Tiny). Does not support attributes. package Chatty; use Role::Tiny; requires 'introduce'; # a method consuming classes must implement sub greet { # a method already implemented in the role print "Hi!\n"; } package Parrot; use Class::Tiny; use Role::Tiny::With; with 'Chatty'; sub introduce { print "I'm Buddy.\n"; } GoalKicker.com – Perl® Notes for Professionals 53 Chapter 17: Exception handling Section 17.1: eval and die This is the built-in way to deal with "exceptions" without relying on third party libraries like Try::Tiny. my $ret; eval { $ret = some_function_that_might_die(); 1; } or do { my $eval_error = $@ || "Zombie error!"; handle_error($eval_error); }; # use $ret We "abuse" the fact that die has a false return value, and the return value of the overall code block is the value of the last expression in the code block: if $ret is assigned to successfully, then the 1; expression is the last thing that happens in the eval code block. The eval code block thus has a true value, so the or do block does not run. if some_function_that_might_die() does die, then the last thing that happens in the eval code block is the die. The eval code block thus has a false value and the or do block does run. The first thing you must do in the or do block is read $@. This global variable will hold whatever argument was passed to die. The || "Zombie Error" guard is popular, but unnecessary in the general case. This is important to understand because some not all code does fail by calling die, but the same structure can be used regardless. Consider a database function that returns: the number of rows affected on success '0 but true' if the query is successful but no rows were affected 0 if the query was not successful. In that case you can still use the same idiom, but you have to skip the final 1;, and this function has to be the last thing in the eval. Something like this: eval { my $value = My::Database::retrieve($my_thing); # dies on fail $value->set_status("Completed"); $value->set_completed_timestamp(time()); $value->update(); # returns false value on fail } or do { # handles both the die and the 0 return value my $eval_error = $@ || "Zombie error!"; handle_error($eval_error); }; GoalKicker.com – Perl® Notes for Professionals 54 Chapter 18: Regular Expressions Section 18.1: Replace a string using regular expressions s/foo/bar/; # replace "foo" with "bar" in $_ my $foo = "foo"; $foo =~ s/foo/bar/; # do the above on a different variable using the binding operator =~ s~ foo ~ bar ~; # using ~ as a delimiter $foo = s/foo/bar/r; # non-destructive r flag: returns the replacement string without modifying the variable it's bound to s/foo/bar/g; # replace all instances Section 18.2: Matching strings The =~ operator attempts to match a regular expression (set apart by /) to a string: my $str = "hello world"; print "Hi, yourself!\n" if $str =~ /^hello/; /^hello/ is the actual regular expression. The ^ is a special character that tells the regular expression to start with the beginning of the string and not match in the middle somewhere. Then the regex tries to find the following letters in order h, e, l, l, and o. Regular expressions attempt to match the default variable ($_) if bare: $_ = "hello world"; print "Ahoy!\n" if /^hello/; You can also use different delimiters is you precede the regular expression with the m operator: m~^hello~; m{^hello}; m|^hello|; This is useful when matching strings that include the / character: print "user directory" if m|^/usr|; Section 18.3: Parsing a string with a regex Generally, it's not a good idea to use a regular expression to parse a complex structure. But it can be done. For instance, you might want to load data into hive table and fields are separated by comma but complex types like array are separated by a "|". Files contain records with all fields separated by comma and complex type are inside square bracket. In that case, this bit of disposable Perl might be sufficient: echo "1,2,[3,4,5],5,6,[7,8],[1,2,34],5" | \ perl -ne \ 'while( /\[[^,\]]+\,.*\]/ ){ if( /\[([^\]\|]+)\]/){ $text = $1; $text_to_replace = $text; $text =~ s/\,/\|/g; s/$text_to_replace/$text/; GoalKicker.com – Perl® Notes for Professionals 55 } } print' You'll want to spot check the output: 1,2,[3|4|5],5,6,[7|8],[1|2|34],5 Section 18.4: Usage of \Q and \E in pattern matching What's between \Q and \E is treated as normal characters #!/usr/bin/perl my $str = "hello.it's.me"; my @test = ( "hello.it's.me", "hello/it's!me", ); sub ismatched($) { $_[0] ? "MATCHED!" : "DID NOT MATCH!" } my @match = ( [ general_match=> sub { ismatched /$str/ } ], [ qe_match => sub { ismatched /\Q$str\E/ } ], ); for (@test) { print "\String = '$_':\n"; foreach my $method (@match) { my($name,$match) = @$method; print " - $name: ", $match->(), "\n"; } } Output String = 'hello.it's.me': - general_match: MATCHED! - qe_match: MATCHED! String = 'hello/it's!me': - general_match: MATCHED! - qe_match: DID NOT MATCH! GoalKicker.com – Perl® Notes for Professionals 56 Chapter 19: XML Parsing Section 19.1: Parsing with XML::Twig #!/usr/bin/env perl use strict; use warnings 'all'; use XML::Twig; my $twig = XML::Twig->parse( \*DATA ); #we can use the 'root' method to find the root of the XML. my $root = $twig->root; #first_child finds the first child element matching a value. my $title = $root->first_child('title'); #text reads the text of the element. my $title_text = $title->text; print "Title is: ", $title_text, "\n"; #The above could be combined: print $twig ->root->first_child_text('title'), "\n"; ## You can use the 'children' method to iterate multiple items: my $list = $twig->root->first_child('list'); #children can optionally take an element 'tag' - otherwise it just returns all of them. foreach my $element ( $list->children ) { #the 'att' method reads an attribute print "Element with ID: ", $element->att('id') // 'none here', " is ", $element->text, "\n"; } #And if we need to do something more complicated, we an use 'xpath'. #get_xpath or findnodes do the same thing: #return a list of matches, or if you specify a second numeric argument, just that numbered match. #xpath syntax is fairly extensive, but in this one - we search: # anywhere in the tree: // #nodes called 'item' #with an id attribute [@id] #and with that id attribute equal to "1000". #by specifying '0' we say 'return just the first match'. print "Item 1000 is: ", $twig->get_xpath( '//item[@id="1000"]', 0 )->text, "\n"; #this combines quite well with `map` to e.g. do the same thing on multiple items print "All IDs:\n", join ( "\n", map { $_ -> att('id') } $twig -> get_xpath('//item')); #note how this also finds the item under 'summary', because of // __DATA__ GoalKicker.com – Perl® Notes for Professionals 57 Some text Text here too Item1 Item2 Item3 Item66 Item88 Item100 Item1000 Not an item at all really.

Test

Section 19.2: Consuming XML with XML::Rabbit With XML::Rabbit it is possible to consume XML files easily. You define in a declarative way and with an XPath syntax what you are looking for in the XML and XML::Rabbit will return objects according to the given definition. Definition: package Bookstore; use XML::Rabbit::Root; has_xpath_object_list books => './book' => 'Bookstore::Book'; finalize_class(); package Bookstore::Book; use XML::Rabbit; has_xpath_value bookid => './@id'; has_xpath_value author => './author'; has_xpath_value title => './title'; has_xpath_value genre => './genre'; has_xpath_value price => './price'; has_xpath_value publish_date => './publish_date'; has_xpath_value description => './description'; has_xpath_object purchase_data => './purchase_data' => 'Bookstore::Purchase'; finalize_class(); package Bookstore::Purchase; use XML::Rabbit; has_xpath_value price => './price'; has_xpath_value date => './date'; finalize_class(); XML Consumption: use strict; use warnings; use utf8; package Library; use feature qw(say); use Carp; use autodie; GoalKicker.com – Perl® Notes for Professionals 58 say "Showing data information"; my $bookstore = Bookstore->new( file => './sample.xml' ); foreach my $book( @{$bookstore->books} ) { say "ID: " . $book->bookid; say "Title: " . $book->title; say "Author: " . $book->author, "\n"; } Notes: Please be careful with the following: 1. The first class has to be XML::Rabbit::Root. It will place you inside the main tag of the XML document. In our case it will place us inside 2. Nested classes which are optional. Those classes need to be accessed via a try/catch (or eval / $@ check) block. Optional fields will simply return null. For example, for purchase_data the loop would be: foreach my $book( @{$bookstore->books} ) { say "ID: " . $book->bookid; say "Title: " . $book->title; say "Author: " . $book->author; try { say "Purchase price: ". $book->purchase_data->price, "\n"; } catch { say "No purchase price available\n"; } } sample.xml Gambardella, Matthew Computer 44.95 2000-10-01 An in-depth look at creating applications with XML. Ralls, Kim Fantasy 5.95 2000-12-16 A former architect battles corporate zombies, an evil sorceress, and her own childhood to become queen of the world. Corets, Eva Fantasy 5.95 2000-11-17 GoalKicker.com – Perl® Notes for Professionals 59 After the collapse of a nanotechnology society in England, the young survivors lay the foundation for a new society. Corets, Eva Fantasy 5.95 2001-03-10 In post-apocalypse England, the mysterious agent known only as Oberon helps to create a new life for the inhabitants of London. Sequel to Maeve Ascendant. 2001-12-21 20 Section 19.3: Parsing with XML::LibXML # This uses the 'sample.xml' given in the XML::Twig example. # Module requirements (1.70 and above for use of load_xml) use XML::LibXML '1.70'; # let's be a good perl dev use strict; use warnings 'all'; # Create the LibXML Document Object my $xml = XML::LibXML->new(); # Where we are retrieving the XML from my $file = 'sample.xml'; # Load the XML from the file my $dom = XML::LibXML->load_xml( location => $file ); # get the docroot my $root = $dom->getDocumentElement; # if the document has children if($root->hasChildNodes) { # getElementsByLocalName returns a node list of all elements who's # localname matches 'title', and we want the first occurrence # (via get_node(1)) my $title = $root->getElementsByLocalName('title'); if(defined $title) { # Get the first matched node out of the nodeList my $node = $title->get_node(1); # Get the text of the target node my $title_text = $node->textContent; GoalKicker.com – Perl® Notes for Professionals 60 print "The first node with name 'title' contains: $title_text\n"; } # The above calls can be combined, but is possibly prone to errors # (if the getElementsByLocalName() failed to match a node). # # my $title_text = $root->getElementsByLocalName('title')->get_node(1)->textContent; } # Using Xpath, get the price of the book with id 'bk104' # # Set our xpath my $xpath = q!/catalog/book[@id='bk104']/price!; # Does that xpath exist? if($root->exists($xpath)) { # Pull in the twig my $match = $root->find($xpath); if(defined $match) { # Get the first matched node out of the nodeList my $node = $match->get_node(1); # pull in the text of that node my $match_text = $node->textContent; print "The price of the book with id bk104 is: $match_text\n"; } } GoalKicker.com – Perl® Notes for Professionals 61 Chapter 20: Unicode Section 20.1: The utf8 pragma: using Unicode in your sources The utf8 pragma indicates that the source code will be interpreted as UTF-8. Of course, this will only work if your text editor is also saving the source as UTF-8 encoded. Now, string literals can contain arbitrary Unicode characters; identifiers can also contain Unicode but only word-like characters (see perldata and perlrecharclass for more information): use utf8; my $var1 = '§?§©????'; # works fine my $? = 4; # works since ? is a word (matches \w) character my $p§2 = 3; # does not work since § is not a word character. say "ya" if $var1 =~ /?§/; # works fine (prints "ya") Note: When printing text to the terminal, make sure it supports UTF-8.* There may be complex and counter-intuitive relationships between output and source encoding. Running on a UTF-8 terminal, you may find that adding the utf8 pragma seems to break things: $ perl -e 'print "Møøse\n"' Møøse $ perl -Mutf8 -e 'print "Møøse\n"' M??se $ perl -Mutf8 -CO -e 'print "Møøse\n"' Møøse In the first case, Perl treats the string as raw bytes and prints them like that. As these bytes happen to be valid UTF-8, they look correct even though Perl doesn't really know what characters they are (e.g. length("Møøse") will return 7, not 5). Once you add -Mutf8, Perl correctly decodes the UTF-8 source to characters, but output is in Latin-1 mode by default and printing Latin-1 to a UTF-8 terminal doesn't work. Only when you switch STDOUT to UTF-8 using -CO will the output be correct. use utf8 doesn't affect standard I/O encoding nor file handles! Section 20.2: Handling invalid UTF-8 Reading invalid UTF-8 When reading UTF-8 encoded data, it is important to be aware of the fact the UTF-8 encoded data can be invalid or malformed. Such data should usually not be accepted by your program (unless you know what you are doing). When unexpectedly encountering malformed data, different actions can be considered: Print stacktrace or error message, and abort program gracefully, or Insert a substitution character at the place where the malformed byte sequence appeared, print a warning message to STDERR and continue reading as nothing happened. By default, Perl will warn you about encoding glitches, but it will not abort your program. You can make your program abort by making UTF-8 warnings fatal, but be aware of the caveats in Fatal Warnings. The following example writes 3 bytes in encoding ISO 8859-1 to disk. It then tries to read the bytes back again as UTF-8 encoded data. One of the bytes, 0xE5, is an invalid UTF-8 one byte sequence: GoalKicker.com – Perl® Notes for Professionals 62 use strict; use warnings; use warnings FATAL => 'utf8'; binmode STDOUT, ':utf8'; binmode STDERR, ':utf8'; my $bytes = "\x{61}\x{E5}\x{61}"; # 3 bytes in iso 8859-1: aåa my $fn = 'test.txt'; open ( my $fh, '>:raw', $fn ) or die "Could not open file '$fn': $!"; print $fh $bytes; close $fh; open ( $fh, "<:encoding(utf-8)", $fn ) or die "Could not open file '$fn': $!"; my $str = do { local $/; <$fh> }; close $fh; print "Read string: '$str'\n"; The program will abort with a fatal warning: utf8 "\xE5" does not map to Unicode at ./test.pl line 10. Line 10 is here the second last line, and the error occurs in the part of the line with <$fh> when trying to read a line from the file. If you don't make warnings fatal in the above program, Perl will still print the warning. However, in this case it will try to recover from the malformed byte 0xE5 by inserting the four characters \xE5 into the stream, and then continue with the next byte. As a result, the program will print: Read string: 'a\xE5a' Section 20.3: Command line switches for one-liners Enable utf8 pragma In order to enable utf8 pragma in one-liner, perl interpreter should be called with -Mutf8 option: perl -Mutf8 -E 'my $人 = "human"; say $人' Unicode handling with -C switch The -C command line flag lets you control Unicode features. It can be followed by a list of option letters. Standard I/O I - STDIN will be in UTF-8 O - STDOUT will be in UTF-8 E - STDERR will be in UTF-8 S - shorthand for IOE, standard I/O streams will be in UTF-8 echo "Ματαιότης ματαιοτήτων" | perl -CS -Mutf8 -nE 'say "ok" if /Ματαιότης/' Script's arguments A - treats @ARGV as an array of UTF-8 encoded strings perl -CA -Mutf8 -E 'my $arg = shift; say "anteater" if $arg eq "муравьед"' муравьед GoalKicker.com – Perl® Notes for Professionals 63 Default PerlIO layer i - UTF-8 is the default PerlIO layer for input streams o - UTF-8 is the default PerlIO layer for output streams D - shorthand for io perl -CD -Mutf8 -e 'open my $fh, ">", "utf8.txt" or die $!; print $fh "개미 조심해"' -M and -C switches may be combined: perl -CASD -Mutf8 -E 'say "Ματαιότης ματαιοτήτων\n"'; Section 20.4: Standard I/O The encoding to be used for the standard I/O filehandles (STDIN, STDOUT, and STDERR), can be set separately for each handle using binmode: binmode STDIN, ':encoding(utf-8)'; binmode STDOUT, ':utf8'; binmode STDERR, ':utf8'; Note: when reading one would in general prefer :encoding(utf-8) over :utf8, see Remarks for more information. Alternatively, you can use the open pragma. # Setup such that all subsequently opened input streams will use ':encoding(utf-8)' # and all subsequently opened output streams will use ':utf8' # by default use open (IN => ':encoding(utf-8)', OUT => ':utf8'); # Make the (already opened) standard file handles inherit the setting # given by the IO settings for the open pragma use open ( :std ); # Now, STDIN has been converted to ':encoding(utf-8)', and # STDOUT and STDERR have ':utf8' Alternatively, to set all filehandles (both those yet to be opened and also the standard ones) to use :encoding(utf-8): use open qw( :encoding(utf-8) :std ); Section 20.5: File handles Setting encoding with open() When opening a text file, you may specify it's encoding explicitly with a three-argument open(). This en-/decoder attached to a file handle is called an "I/O layer": my $filename = '/path/to/file'; open my $fh, '<:encoding(utf-8)', $filename or die "Failed to open $filename: $!"; See Remarks for a discussion of the differences between :utf8 and :encoding(utf-8). Setting encoding with binmode() Alternatively, you may use binmode() to set the encoding for individual file handle: GoalKicker.com – Perl® Notes for Professionals 64 my $filename = '/path/to/file'; open my $fh, '<', $filename or die "Failed to open $filename: $!"; binmode $fh, ':encoding(utf-8)'; open pragma To avoid setting encoding for each file handle separately, you may use the open pragma to set a default I/O layer used by all subsequent calls to the open() function and similar operators within the lexical scope of this pragma: # Set input streams to ':encoding(utf-8)' and output streams to ':utf8' use open (IN => ':encoding(utf-8)', OUT => ':utf8'); # Or to set all input and output streams to ':encoding(utf-8)' use open ':encoding(utf-8)'; Setting encoding with command line -C flag Finally, it is also possible to run the perl interpreter with a -CD flag that applies UTF-8 as the default I/O layer. However, this option should be avoided since it relies on specific user behaviour which cannot be predicted nor controlled. Section 20.6: Create filenames The following examples use the UTF-8 encoding to represent filenames (and directory names) on disk. If you want to use another encoding, you should use Encode::encode(...). use v5.14; # Make Perl recognize UTF-8 encoded characters in literal strings. # For this to work: Make sure your text-editor is using UTF-8, so # that bytes on disk are really UTF-8 encoded. use utf8; # Ensure that possible error messages printed to screen are converted to UTF-8. # For this to work: Check that your terminal emulator is using UTF-8. binmode STDOUT, ':utf8'; binmode STDERR, ':utf8'; my $filename = 'æ€'; # $filename is now an internally UTF-8 encoded string. # Note: in the following it is assumed that $filename has the internal UTF-8 # flag set, if $filename is pure ASCII, it will also work since its encoding # overlaps with UTF-8. However, if it has another encoding like extended ASCII, # $filename will be written with that encoding and not UTF-8. # Note: it is not necessary to encode $filename as UTF-8 here # since Perl is using UTF-8 as its internal encoding of $filename already # Example1 -- using open() open ( my $fh, '>', $filename ) or die "Could not open '$filename': $!"; close $fh; # Example2 -- using qx() and touch qx{touch $filename}; # Example3 -- using system() and touch system 'touch', $filename; # Example4 -- using File::Touch use File::Touch; eval { touch( $filename ) }; die "Could not create file '$filename': $!" if $@; GoalKicker.com – Perl® Notes for Professionals 65 Section 20.7: Read filenames Perl does not attempt to decode filenames returned by builtin functions or modules. Such strings representing filenames should always be decoded explicitly, in order for Perl to recognize them as Unicode. use v5.14; use Encode qw(decode_utf8); # Ensure that possible error messages printed to screen are converted to UTF-8. # For this to work: Check that you terminal emulator is using UTF-8. binmode STDOUT, ':utf8'; binmode STDERR, ':utf8'; # Example1 -- using readdir() my $dir = '.'; opendir(my $dh, $dir) or die "Could not open directory '$dir': $!"; while (my $filename = decode_utf8(readdir $dh)) { # Do something with $filename } close $dh; # Example2 -- using getcwd() use Cwd qw(getcwd); my $dir = decode_utf8( getcwd() ); # Example3 -- using abs2rel() use File::Spec; use utf8; my $base = 'ø'; my $path = "$base/b/æ"; my $relpath = decode_utf8( File::Spec->abs2rel( $path, $base ) ); # Note: If you omit $base, you need to encode $path first: use Encode qw(encode_utf8); my $relpath = decode_utf8( File::Spec->abs2rel( encode_utf8( $path ) ) ); # Example4 -- using File::Find::Rule (part1 matching a filename) use File::Find::Rule; use utf8; use Encode qw(encode_utf8); my $filename = 'æ'; # File::Find::Rule needs $filename to be encoded my @files = File::Find::Rule->new->name( encode_utf8($filename) )->in('.'); $_ = decode_utf8( $_ ) for @files; # Example5 -- using File::Find::Rule (part2 matching a regular expression) use File::Find::Rule; use utf8; my $pat = '[æ].$'; # Unicode pattern # Note: In this case: File::Find::Rule->new->name( qr/$pat/ )->in('.') # will not work since $pat is Unicode and filenames are bytes # Also encoding $pat first will not work correctly my @files; File::Find::Rule->new->exec( sub { wanted( $pat, \@files ) } )->in('.'); $_ = decode_utf8( $_ ) for @files; sub wanted { my ( $pat, $files ) = @_; my $name = decode_utf8( $_ ); my $full_name = decode_utf8( $File::Find::name ); push @$files, $full_name if $name =~ /$pat/; } GoalKicker.com – Perl® Notes for Professionals 66 Note: if you are concerned about invalid UTF-8 in the filenames, the use of decode_utf8( ... ) in the above examples should probably be replaced by decode( 'utf-8', ... ). This is because decode_utf8( ... ) is a synonym for decode( 'utf8', ... ) and there is a difference between the encodings utf-8 and utf8 (see Remarks below for more information) where utf-8 is more strict on what is acceptable than utf8. GoalKicker.com – Perl® Notes for Professionals 67 Chapter 21: Perl one-liners Section 21.1: Upload file into mojolicious perl -Mojo -E 'p("http://localhost:3000" => form => {Input_Type => "XML", Input_File => {file => "d:/xml/test.xml"}})' File d:/xml/test.xml will be uploaded to server which listen connections on localhost:3000 (Source) In this example: -Mmodule executes use module; before executing your program -E commandline is used to enter one line of program If you have no ojo module you can use cpanm ojo command to install it To read more about how to run perl use perldoc perlrun command or read here Section 21.2: Execute some Perl code from command line Simple one-liners may be specified as command line arguments to perl using the -e switch (think "execute"): perl -e'print "Hello, World!\n"' Due to Windows quoting rules you can't use single-quoted strings but have to use one of these variants: perl -e"print qq(Hello, World!\n)" perl -e"print \"Hello, World!\n\"" Note that to avoid breaking old code, only syntax available up to Perl 5.8.x can be used with -e. To use anything newer your perl version may support, use -E instead. E.g. to use say available from 5.10.0 on plus Unicode 6.0 from >=v5.14.0 (also uses -CO to make sure STDOUT prints UTF-8): Version ≥ 5.14.0 perl -CO -E'say "\N{PILE OF POO}"' Section 21.3: Using double-quoted strings in Windows one liners Windows uses only double quotes to wrap command line parameters. In order to use double quotes in perl one liner (i.e. to print a string with an interpolated variable), you have to escape them with backslashes: perl -e "my $greeting = 'Hello'; print \"$greeting, world!\n\"" To improve readability, you may use a qq() operator: perl -e "my $greeting = 'Hello'; print qq($greeting, world!\n)" Section 21.4: Print lines matching a pattern (PCRE grep) perl -ne'print if /foo/' file.txt Case-insensitive: GoalKicker.com – Perl® Notes for Professionals 68 perl -ne'print if /foo/i' file.txt Section 21.5: Replace a substring with another (PCRE sed) perl -pe"s/foo/bar/g" file.txt Or in-place: perl -i -pe's/foo/bar/g' file.txt On Windows: perl -i.bak -pe"s/foo/bar/g" file.txt Section 21.6: Print only certain fields perl -lane'print "$F[0] $F[-1]"' data.txt # prints the first and the last fields of a space delimited record CSV example: perl -F, -lane'print "$F[0] $F[-1]"' data.csv Section 21.7: Print lines 5 to 10 perl -ne'print if 5..10' file.txt Section 21.8: Edit file in-place Without a backup copy (not supported on Windows) perl -i -pe's/foo/bar/g' file.txt With a backup copy file.txt.bak perl -i.bak -pe's/foo/bar/g' file.txt With a backup copy old_file.txt.orig in the backup subdirectory (provided the latter exists): perl -i'backup/old_*.orig' -pe's/foo/bar/g' file.txt Section 21.9: Reading the whole file as a string perl -0777 -ne'print "The whole file as a string: --->$_<---\n"' Note: The -0777 is just a convention. Any -0400 and above would de the same. GoalKicker.com – Perl® Notes for Professionals 69 Chapter 22: Randomness Section 22.1: Accessing an array element at random my @letters = ( 'a' .. 'z' ); # English ascii-bet print $letters[ rand @letters ] for 1 .. 5; # prints 5 letters at random How it works rand EXPR expects a scalar value, so @letters is evaluated in scalar context An array in scalar context returns the number of elements it contains (26 in this case) rand 26 returns a random fractional number in the interval 0 ≤ VALUE < 26. (It can never be 26) Array indices are always integers, so $letters[rand @letters] ≡ $letters[int rand @letters] Perl arrays are zero-indexed, so $array[rand @array] returns $array[0], $array[$#array] or an element in between (The same principle applies to hashes) my %colors = ( red => 0xFF0000, green => 0x00FF00, blue => 0x0000FF, ); print ( values %colors )[rand keys %colors]; Section 22.2: Generate a random integer between 0 and 9 Cast your random floating-point number as an int. Input: my $range = 10; # create random integer as low as 0 and as high as 9 my $random = int(rand($range)); # max value is up to but not equal to $range print $random . "\n"; Output: A random integer, like... 0 See also the perldoc for rand. GoalKicker.com – Perl® Notes for Professionals 70 Chapter 23: Special variables Section 23.1: Special variables in perl: 1. $_ : The default input and pattern-searching space. Example 1: my @array_variable = (1 2 3 4); foreach (@array_variable){ print $_."\n"; # $_ will get the value 1,2,3,4 in loop, if no other variable is supplied. } Example 2: while (){ chomp($_); # $_ refers to the iterating lines in the loop. } The following functions use $_ as a default argument: abs, alarm, chomp, chop, chr, chroot, cos, defined, eval, evalbytes, exp, fc, glob, hex, int, lc, lcfirst, length, log, lstat, mkdir, oct, ord, pos, print, printf, quotemeta, readlink, readpipe, ref, require, reverse (in scalar context only), rmdir, say, sin, split (for its second argument), sqrt, stat, study, uc, ucfirst, unlink, unpack. 2. @_ : This array contains the arguments passed to subroutine. Example 1: example_sub( $test1, $test2, $test3 ); sub example_sub { my ( $test1, $test2, $test3 ) = @_; } Within a subroutine the array @_ contains the arguments passed to that subroutine. Inside a subroutine, @_ is the default array for the array operators pop and shift. GoalKicker.com – Perl® Notes for Professionals 71 Chapter 24: Packages and modules Section 24.1: Using a module use Cwd; This will import the Cwd module at compile time and import its default symbols, i.e. make some of the module's variables and functions available to the code using it. (See also: perldoc -f use.) Generally this is will do the right thing. Sometimes, however, you will want to control which symbols are imported. Add a list of symbols after the module name to export: use Cwd 'abs_path'; If you do this, only the symbols you specify will be imported (ie, the default set will not be imported). When importing multiple symbols, it is idiomatic to use the qw() list-building construct: use Cwd qw(abs_path realpath); Some modules export a subset of their symbols, but can be told to export everything with :all: use Benchmark ':all'; (Note that not all modules recognize or use the :all tag). Section 24.2: Using a module inside a directory use lib 'includes'; use MySuperCoolModule; use lib 'includes'; adds the relative directory includes/ as another module search path in @INC. So assume that you have a module file MySyperCoolModule.pm inside includes/, which contains: package MySuperCoolModule; If you want, you can group as many modules of your own inside a single directory and make them findable with one use lib statement. At this point, using the subroutines in the module will require prefixing the subroutine name with the package name: MySuperCoolModule::SuperCoolSub_1("Super Cool String"); To be able to use the subroutines without the prefix, you need to export the subroutine names so that they are recognised by the program calling them. Exporting can be set up to be automatic, thus: package MySuperCoolModule; use base 'Exporter'; our @EXPORT = ('SuperCoolSub_1', 'SuperCoolSub_2'); Then in the file that uses the module, those subroutines will be automatically available: GoalKicker.com – Perl® Notes for Professionals 72 use MySuperCoolModule; SuperCoolSub_1("Super Cool String"); Or you can set up the module to conditionally export subroutines, thus: package MySuperCoolModule; use base 'Exporter'; our @EXPORT_OK = ('SuperCoolSub_1', 'SuperCoolSub_2'); In which case, you need to explicitly request the desired subroutines to be exported in the script that uses the module: use MySuperCoolModule 'SuperCoolSub_1'; SuperCoolSub_1("Super Cool String"); Section 24.3: Loading a module at runtime require Exporter; This will ensure that the Exporter module is loaded at runtime if it hasn't already been imported. (See also: perldoc -f require.) N.B.: Most users should use modules rather than require them. Unlike use, require does not call the module's import method and is executed at runtime, not during the compile. This way of loading modules is useful if you can't decide what modules you need before runtime, such as with a plugin system: package My::Module; my @plugins = qw( One Two ); foreach my $plugin (@plugins) { my $module = __PACKAGE__ . "::Plugins::$plugin"; $module =~ s!::!/!g; require "$module.pm"; } This would try to load My::Package::Plugins::One and My::Package::Plugins::Two. @plugins should of course come from some user input or a config file for this to make sense. Note the substitution operator s!::!/!g that replaces each pair of colons with a slash. This is because you can load modules using the familiar module name syntax from use only if the module name is a bareword. If you pass a string or a variable, it must contain a file name. Section 24.4: CPAN.pm CPAN.pm is a Perl module which allows to query and install modules from CPAN sites. It supports interactive mode invoked with cpan or perl -MCPAN -e shell GoalKicker.com – Perl® Notes for Professionals 73 Querying modules By name: cpan> m MooseX::YAML By a regex against module name: cpan> m /^XML::/ Note: to enable a pager or redirecting to a file use | or > shell redirection (spaces are mandatory around the | and >), e.g.: m /^XML::/ | less. By distribution: cpan> d LMC/Net-Squid-Auth-Engine-0.04.tar.gz Installing modules By name: cpan> install MooseX::YAML By distribution: cpan> install LMC/Net-Squid-Auth-Engine-0.04.tar.gz Section 24.5: List all installed modules From command line: cpan -l From a Perl script: use ExtUtils::Installed; my $inst = ExtUtils::Installed->new(); my @modules = $inst->modules(); Section 24.6: Executing the contents of another file do './config.pl'; This will read in the contents of the config.pl file and execute it. (See also: perldoc -f do.) N.B.: Avoid do unless golfing or something as there is no error checking. For including library modules, use require or use. GoalKicker.com – Perl® Notes for Professionals 74 Chapter 25: Install Perl modules via CPAN Section 25.1: cpanminus, the lightweight configuration-free replacement for cpan Usage To install a module (assuming cpanm is already installed): cpanm Data::Section cpanm ("cpanminus") strives to be less verbose than cpan but still captures all of the installation information in a log file in case it is needed. It also handles many "interactive questions" for you, whereas cpan doesn't. cpanm is also popular for installing dependencies of a project from, e.g., GitHub. Typical use is to first cd into the project's root, then run cpanm --installdeps . With --installdeps it will: 1. Scan and install configure_requires dependencies from either META.json META.yml (if META.json is missing) 2. Build the project (equivalent to perl Build.PL), generating MYMETA files 3. Scan and install requires dependencies from either MYMETA.json MYMETA.yml (if MYMETA.json is missing) To specify the file 'some.cpanfile', containing the dependencies, run: cpanm --installdeps --cpanfile some.cpanfile . cpanm Installation There are several ways to install it. Here's installation via cpan: cpan App::cpanminus cpanm Configuration There is no config file for cpanm. Rather, it relies on the following environment variables for its configuration: PERL_CPANM_OPT (General cpanm command line options) export PERL_CPANM_OPT="--prompt" # in .bashrc, to enable prompting, e.g. setenv PERL_CPANM_OPT "--prompt" # in .tcshrc PERL_MM_OPT (ExtUtils::MakeMaker command line options, affects module install target) PERL_MB_OPT (Module::Build command line options, affects module install target) Section 25.2: Installing modules manually If you don't have permissions to install perl modules, you may still install them manually, indicating a custom path where you've got writing permissions. 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