PERLINTRO(1) Perl Programmers Reference Guide PERLINTRO(1)

PERLINTRO(1) Perl Programmers Reference Guide PERLINTRO(1) #

PERLINTRO(1) Perl Programmers Reference Guide PERLINTRO(1)

NNAAMMEE #

 perlintro - a brief introduction and overview of Perl

DDEESSCCRRIIPPTTIIOONN #

 This document is intended to give you a quick overview of the Perl
 programming language, along with pointers to further documentation.  It
 is intended as a "bootstrap" guide for those who are new to the language,
 and provides just enough information for you to be able to read other
 peoples' Perl and understand roughly what it's doing, or write your own
 simple scripts.

 This introductory document does not aim to be complete.  It does not even
 aim to be entirely accurate.  In some cases perfection has been
 sacrificed in the goal of getting the general idea across.  You are
 _s_t_r_o_n_g_l_y advised to follow this introduction with more information from
 the full Perl manual, the table of contents to which can be found in
 perltoc.

 Throughout this document you'll see references to other parts of the Perl
 documentation.  You can read that documentation using the "perldoc"
 command or whatever method you're using to read this document.

 Throughout Perl's documentation, you'll find numerous examples intended
 to help explain the discussed features.  Please keep in mind that many of
 them are code fragments rather than complete programs.

 These examples often reflect the style and preference of the author of
 that piece of the documentation, and may be briefer than a corresponding
 line of code in a real program.  Except where otherwise noted, you should
 assume that "use strict" and "use warnings" statements appear earlier in
 the "program", and that any variables used have already been declared,
 even if those declarations have been omitted to make the example easier
 to read.

 Do note that the examples have been written by many different authors
 over a period of several decades.  Styles and techniques will therefore
 differ, although some effort has been made to not vary styles too widely
 in the same sections.  Do not consider one style to be better than others
 - "There's More Than One Way To Do It" is one of Perl's mottos.  After
 all, in your journey as a programmer, you are likely to encounter
 different styles.

WWhhaatt iiss PPeerrll?? Perl is a general-purpose programming language originally developed for text manipulation and now used for a wide range of tasks including system administration, web development, network programming, GUI development, and more.

 The language is intended to be practical (easy to use, efficient,
 complete) rather than beautiful (tiny, elegant, minimal).  Its major
 features are that it's easy to use, supports both procedural and object-
 oriented (OO) programming, has powerful built-in support for text
 processing, and has one of the world's most impressive collections of
 third-party modules.

 Different definitions of Perl are given in perl, perlfaq1 and no doubt
 other places.  From this we can determine that Perl is different things
 to different people, but that lots of people think it's at least worth
 writing about.

RRuunnnniinngg PPeerrll pprrooggrraammss To run a Perl program from the Unix command line:

  perl progname.pl

 Alternatively, put this as the first line of your script:

  #!/usr/bin/env perl

 ... and run the script as _/_p_a_t_h_/_t_o_/_s_c_r_i_p_t_._p_l.  Of course, it'll need to
 be executable first, so "chmod 755 script.pl" (under Unix).

 (This start line assumes you have the eennvv program.  You can also put
 directly the path to your perl executable, like in "#!/usr/bin/perl").

 For more information, including instructions for other platforms such as
 Windows, read perlrun.

SSaaffeettyy nneett Perl by default is very forgiving. In order to make it more robust it is recommended to start every program with the following lines:

  #!/usr/bin/perl
  use strict;
  use warnings;

 The two additional lines request from perl to catch various common
 problems in your code.  They check different things so you need both.  A
 potential problem caught by "use strict;" will cause your code to stop
 immediately when it is encountered, while "use warnings;" will merely
 give a warning (like the command-line switch --ww) and let your code run.
 To read more about them, check their respective manual pages at strict
 and warnings.

 A "use v5.35" (or higher) declaration will enable both "strict" and
 "warnings":

   #!/usr/bin/perl
   use v5.35;

 In addition to enabling the "strict" and "warnings" pragmata, this
 declaration will also activate a "feature bundle"; a collection of named
 features that enable many of the more recent additions and changes to the
 language, as well as occasionally removing older features found to have
 been mistakes in design and discouraged.

BBaassiicc ssyynnttaaxx oovveerrvviieeww A Perl script or program consists of one or more statements. These statements are simply written in the script in a straightforward fashion. There is no need to have a “main()” function or anything of that kind.

 Perl statements end in a semi-colon:

  print "Hello, world";

 Comments start with a hash symbol and run to the end of the line

  # This is a comment

 Whitespace is irrelevant:

  print
      "Hello, world"
      ;

 ... except inside quoted strings:

  # this would print with a linebreak in the middle
  print "Hello
  world";

 Double quotes or single quotes may be used around literal strings:

  print "Hello, world";
  print 'Hello, world';

 However, only double quotes "interpolate" variables and special
 characters such as newlines ("\n"):

  print "Hello, $name\n";     # works fine
  print 'Hello, $name\n';     # prints $name\n literally

 Numbers don't need quotes around them:

  print 42;

 You can use parentheses for functions' arguments or omit them according
 to your personal taste.  They are only required occasionally to clarify
 issues of precedence.

  print("Hello, world\n");
  print "Hello, world\n";

 More detailed information about Perl syntax can be found in perlsyn.

PPeerrll vvaarriiaabbllee ttyyppeess Perl has three main variable types: scalars, arrays, and hashes.

 Scalars
     A scalar represents a single value:

      my $animal = "camel";
      my $answer = 42;

     Scalar values can be strings, integers or floating point numbers, and
     Perl will automatically convert between them as required.  You have
     to declare them using the "my" keyword the first time you use them.
     (This is one of the requirements of "use strict;".)

     Scalar values can be used in various ways:

      print $animal;
      print "The animal is $animal\n";
      print "The square of $answer is ", $answer * $answer, "\n";

     Perl defines a number of special scalars with short names, often
     single punctuation marks or digits. These variables are used for all
     kinds of purposes, and are documented in perlvar.  The only one you
     need to know about for now is $_ which is the "default variable".
     It's used as the default argument to a number of functions in Perl,
     and it's set implicitly by certain looping constructs.

      print;          # prints contents of $_ by default

 Arrays
     An array represents a list of values:

      my @animals = ("camel", "llama", "owl");
      my @numbers = (23, 42, 69);
      my @mixed   = ("camel", 42, 1.23);

     Arrays are zero-indexed.  Here's how you get at elements in an array:

      print $animals[0];              # prints "camel"
      print $animals[1];              # prints "llama"

     The special variable $#array tells you the index of the last element
     of an array:

      print $mixed[$#mixed];       # last element, prints 1.23

     You might be tempted to use "$#array + 1" to tell you how many items
     there are in an array.  Don't bother.  As it happens, using @array
     where Perl expects to find a scalar value ("in scalar context") will
     give you the number of elements in the array:

      if (@animals < 5) { ... }

     The elements we're getting from the array start with a "$" because
     we're getting just a single value out of the array; you ask for a
     scalar, you get a scalar.

     To get multiple values from an array:

      @animals[0,1];                 # gives ("camel", "llama");
      @animals[0..2];                # gives ("camel", "llama", "owl");
      @animals[1..$#animals];        # gives all except the first element

     This is called an "array slice".

     You can do various useful things to lists:

      my @sorted    = sort @animals;
      my @backwards = reverse @numbers;

     There are a couple of special arrays too, such as @ARGV (the command
     line arguments to your script) and @_ (the arguments passed to a
     subroutine).  These are documented in perlvar.

 Hashes
     A hash represents a set of key/value pairs:

      my %fruit_color = ("apple", "red", "banana", "yellow");

     You can use whitespace and the "=>" operator to lay them out more
     nicely:

      my %fruit_color = (
          apple  => "red",
          banana => "yellow",
      );

     To get at hash elements:

      $fruit_color{"apple"};           # gives "red"

     You can get at lists of keys and values with "keys()" and "values()".

      my @fruits = keys %fruit_color;
      my @colors = values %fruit_color;

     Hashes have no particular internal order, though you can sort the
     keys and loop through them.

     Just like special scalars and arrays, there are also special hashes.
     The most well known of these is %ENV which contains environment
     variables.  Read all about it (and other special variables) in
     perlvar.

 Scalars, arrays and hashes are documented more fully in perldata.

 More complex data types can be constructed using references, which allow
 you to build lists and hashes within lists and hashes.

 A reference is a scalar value and can refer to any other Perl data type.
 So by storing a reference as the value of an array or hash element, you
 can easily create lists and hashes within lists and hashes.  The
 following example shows a 2 level hash of hash structure using anonymous
 hash references.

  my $variables = {
      scalar  =>  {
                   description => "single item",
                   sigil => '$',
                  },
      array   =>  {
                   description => "ordered list of items",
                   sigil => '@',
                  },
      hash    =>  {
                   description => "key/value pairs",
                   sigil => '%',
                  },
  };

  print "Scalars begin with a $variables->{'scalar'}->{'sigil'}\n";

 Exhaustive information on the topic of references can be found in
 perlreftut, perllol, perlref and perldsc.

VVaarriiaabbllee ssccooppiinngg Throughout the previous section all the examples have used the syntax:

  my $var = "value";

 The "my" is actually not required; you could just use:

  $var = "value";

 However, the above usage will create global variables throughout your
 program, which is bad programming practice.  "my" creates lexically
 scoped variables instead.  The variables are scoped to the block (i.e. a
 bunch of statements surrounded by curly-braces) in which they are
 defined.

  my $x = "foo";
  my $some_condition = 1;
  if ($some_condition) {
      my $y = "bar";
      print $x;           # prints "foo"
      print $y;           # prints "bar"
  }
  print $x;               # prints "foo"
  print $y;               # prints nothing; $y has fallen out of scope

 Using "my" in combination with a "use strict;" at the top of your Perl
 scripts means that the interpreter will pick up certain common
 programming errors.  For instance, in the example above, the final "print
 $y" would cause a compile-time error and prevent you from running the
 program.  Using "strict" is highly recommended.

CCoonnddiittiioonnaall aanndd llooooppiinngg ccoonnssttrruuccttss Perl has most of the usual conditional and looping constructs.

 The conditions can be any Perl expression.  See the list of operators in
 the next section for information on comparison and boolean logic
 operators, which are commonly used in conditional statements.

 if
      if ( condition ) {
          ...
      } elsif ( other condition ) {
          ...
      } else {
          ...
      }

     There's also a negated version of it:

      unless ( condition ) {
          ...
      }

     This is provided as a more readable version of "if (!_c_o_n_d_i_t_i_o_n)".

     Note that the braces are required in Perl, even if you've only got
     one line in the block.  However, there is a clever way of making your
     one-line conditional blocks more English like:

      # the traditional way
      if ($zippy) {
          print "Yow!";
      }

      # the Perlish post-condition way
      print "Yow!" if $zippy;
      print "We have no bananas" unless $bananas;

 while
      while ( condition ) {
          ...
      }

     There's also a negated version, for the same reason we have "unless":

      until ( condition ) {
          ...
      }

     You can also use "while" in a post-condition:

      print "LA LA LA\n" while 1;          # loops forever

 for Exactly like C:

      for ($i = 0; $i <= $max; $i++) {
          ...
      }

     The C style for loop is rarely needed in Perl since Perl provides the
     more friendly list scanning "foreach" loop.

 foreach
      foreach (@array) {
          print "This element is $_\n";
      }

      print $list[$_] foreach 0 .. $max;

      # you don't have to use the default $_ either...
      foreach my $key (keys %hash) {
          print "The value of $key is $hash{$key}\n";
      }

     The "foreach" keyword is actually a synonym for the "for" keyword.
     See ""Foreach Loops" in perlsyn".

 For more detail on looping constructs (and some that weren't mentioned in
 this overview) see perlsyn.

BBuuiillttiinn ooppeerraattoorrss aanndd ffuunnccttiioonnss Perl comes with a wide selection of builtin functions. Some of the ones we’ve already seen include “print”, “sort” and “reverse”. A list of them is given at the start of perlfunc and you can easily read about any given function by using “perldoc -f _f_u_n_c_t_i_o_n_n_a_m_e”.

 Perl operators are documented in full in perlop, but here are a few of
 the most common ones:

 Arithmetic
      +   addition
      -   subtraction
      *   multiplication
      /   division

 Numeric comparison
      ==  equality
      !=  inequality
      <   less than
      >   greater than
      <=  less than or equal
      >=  greater than or equal

 String comparison
      eq  equality
      ne  inequality
      lt  less than
      gt  greater than
      le  less than or equal
      ge  greater than or equal

     (Why do we have separate numeric and string comparisons?  Because we
     don't have special variable types, and Perl needs to know whether to
     sort numerically (where 99 is less than 100) or alphabetically (where
     100 comes before 99).

 Boolean logic
      &&  and
      ||  or
      !   not

     ("and", "or" and "not" aren't just in the above table as descriptions
     of the operators.  They're also supported as operators in their own
     right.  They're more readable than the C-style operators, but have
     different precedence to "&&" and friends.  Check perlop for more
     detail.)

 Miscellaneous
      =   assignment
      .   string concatenation
      x   string multiplication (repeats strings)
      ..  range operator (creates a list of numbers or strings)

 Many operators can be combined with a "=" as follows:

  $a += 1;        # same as $a = $a + 1
  $a -= 1;        # same as $a = $a - 1
  $a .= "\n";     # same as $a = $a . "\n";

FFiilleess aanndd II//OO You can open a file for input or output using the “open()” function. It’s documented in extravagant detail in perlfunc and perlopentut, but in short:

  open(my $in,  "<",  "input.txt")  or die "Can't open input.txt: $!";
  open(my $out, ">",  "output.txt") or die "Can't open output.txt: $!";
  open(my $log, ">>", "my.log")     or die "Can't open my.log: $!";

 You can read from an open filehandle using the "<>" operator.  In scalar
 context it reads a single line from the filehandle, and in list context
 it reads the whole file in, assigning each line to an element of the
 list:

  my $line  = <$in>;
  my @lines = <$in>;

 Reading in the whole file at one time is called slurping.  It can be
 useful but it may be a memory hog.  Most text file processing can be done
 a line at a time with Perl's looping constructs.

 The "<>" operator is most often seen in a "while" loop:

  while (<$in>) {     # assigns each line in turn to $_
      print "Just read in this line: $_";
  }

 We've already seen how to print to standard output using "print()".
 However, "print()" can also take an optional first argument specifying
 which filehandle to print to:

  print STDERR "This is your final warning.\n";
  print $out $record;
  print $log $logmessage;

 When you're done with your filehandles, you should "close()" them (though
 to be honest, Perl will clean up after you if you forget):

  close $in or die "$in: $!";

RReegguullaarr eexxpprreessssiioonnss Perl’s regular expression support is both broad and deep, and is the subject of lengthy documentation in perlrequick, perlretut, and elsewhere. However, in short:

 Simple matching
      if (/foo/)       { ... }  # true if $_ contains "foo"
      if ($a =~ /foo/) { ... }  # true if $a contains "foo"

     The "//" matching operator is documented in perlop.  It operates on
     $_ by default, or can be bound to another variable using the "=~"
     binding operator (also documented in perlop).

 Simple substitution
      s/foo/bar/;               # replaces foo with bar in $_
      $a =~ s/foo/bar/;         # replaces foo with bar in $a
      $a =~ s/foo/bar/g;        # replaces ALL INSTANCES of foo with bar
                                # in $a

     The "s///" substitution operator is documented in perlop.

 More complex regular expressions
     You don't just have to match on fixed strings.  In fact, you can
     match on just about anything you could dream of by using more complex
     regular expressions.  These are documented at great length in perlre,
     but for the meantime, here's a quick cheat sheet:

      .                   a single character
      \s                  a whitespace character (space, tab, newline,
                          ...)
      \S                  non-whitespace character
      \d                  a digit (0-9)
      \D                  a non-digit
      \w                  a word character (a-z, A-Z, 0-9, _)
      \W                  a non-word character
      [aeiou]             matches a single character in the given set
      [^aeiou]            matches a single character outside the given
                          set
      (foo|bar|baz)       matches any of the alternatives specified

      ^                   start of string
      $                   end of string

     Quantifiers can be used to specify how many of the previous thing you
     want to match on, where "thing" means either a literal character, one
     of the metacharacters listed above, or a group of characters or
     metacharacters in parentheses.

      *                   zero or more of the previous thing
      +                   one or more of the previous thing
      ?                   zero or one of the previous thing
      {3}                 matches exactly 3 of the previous thing
      {3,6}               matches between 3 and 6 of the previous thing
      {3,}                matches 3 or more of the previous thing

     Some brief examples:

      /^\d+/              string starts with one or more digits
      /^$/                nothing in the string (start and end are
                          adjacent)
      /(\d\s){3}/         three digits, each followed by a whitespace
                          character (eg "3 4 5 ")
      /(a.)+/             matches a string in which every odd-numbered
                          letter is a (eg "abacadaf")

      # This loop reads from STDIN, and prints non-blank lines:
      while (<>) {
          next if /^$/;
          print;
      }

 Parentheses for capturing
     As well as grouping, parentheses serve a second purpose.  They can be
     used to capture the results of parts of the regexp match for later
     use.  The results end up in $1, $2 and so on.

      # a cheap and nasty way to break an email address up into parts

      if ($email =~ /([^@]+)@(.+)/) {
          print "Username is $1\n";
          print "Hostname is $2\n";
      }

 Other regexp features
     Perl regexps also support backreferences, lookaheads, and all kinds
     of other complex details.  Read all about them in perlrequick,
     perlretut, and perlre.

WWrriittiinngg ssuubbrroouuttiinneess Writing subroutines is easy:

  sub logger {
     my $logmessage = shift;
     open my $logfile, ">>", "my.log" or die "Could not open my.log: $!";
     print $logfile $logmessage;
  }

 Now we can use the subroutine just as any other built-in function:

  logger("We have a logger subroutine!");

 What's that "shift"?  Well, the arguments to a subroutine are available
 to us as a special array called @_ (see perlvar for more on that).  The
 default argument to the "shift" function just happens to be @_.  So "my
 $logmessage = shift;" shifts the first item off the list of arguments and
 assigns it to $logmessage.

 We can manipulate @_ in other ways too:

  my ($logmessage, $priority) = @_;       # common
  my $logmessage = $_[0];                 # uncommon, and ugly

 Subroutines can also return values:

  sub square {
      my $num = shift;
      my $result = $num * $num;
      return $result;
  }

 Then use it like:

  $sq = square(8);

 For more information on writing subroutines, see perlsub.

OOOO PPeerrll OO Perl is relatively simple and is implemented using references which know what sort of object they are based on Perl’s concept of packages. However, OO Perl is largely beyond the scope of this document. Read perlootut and perlobj.

 As a beginning Perl programmer, your most common use of OO Perl will be
 in using third-party modules, which are documented below.

UUssiinngg PPeerrll mmoodduulleess Perl modules provide a range of features to help you avoid reinventing the wheel, and can be downloaded from CPAN ( http://www.cpan.org/ ). A number of popular modules are included with the Perl distribution itself.

 Categories of modules range from text manipulation to network protocols
 to database integration to graphics.  A categorized list of modules is
 also available from CPAN.

 To learn how to install modules you download from CPAN, read
 perlmodinstall.

 To learn how to use a particular module, use "perldoc _M_o_d_u_l_e_:_:_N_a_m_e".
 Typically you will want to "use _M_o_d_u_l_e_:_:_N_a_m_e", which will then give you
 access to exported functions or an OO interface to the module.

 perlfaq contains questions and answers related to many common tasks, and
 often provides suggestions for good CPAN modules to use.

 perlmod describes Perl modules in general.  perlmodlib lists the modules
 which came with your Perl installation.

 If you feel the urge to write Perl modules, perlnewmod will give you good
 advice.

AAUUTTHHOORR #

 Kirrily "Skud" Robert <skud@cpan.org>

perl v5.36.3 2023-02-15 PERLINTRO(1)