مقدمة في بيرل للمبتدئين

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 strongly 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.

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.

لتشغيل برامج بيرل من Unix command line:

  perl progname.pl

أيضاً يمكن وضع الآتي في أول سطر من البرنامج:

  #!/usr/bin/env perl

… و من ثم تشغيل البرنامج بـ path/to/script.pl. بالطبع يحتاج البرنامج قبل هذا أن يكون تنفيذيا (executable)، أي chmod 755 script.pl (في يونكس).

لمزيدٍ من المعلومات, و للإرشادات لمنصات أخرى أخرى مثل Windows و Mac OS، اقرأ دليل perlrun.

برنامج أو سكريبت بيرل يتكون من جملة واحدة أو أكثر. 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.

جمل بيرل تنتهي بفاصلة منقوطة semi-colon.

  print "Hello, world";

التعليقات تبدأ بعلامة مربع و تستمر إلى نهاية السطر

  # 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.

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

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. There is no need to pre-declare your variable types. 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";

  There are a number of magic scalars with names that look like
  punctuation or line noise.  These special 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

المصفوفات تمثل قائمة من القيم:

      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. يمكنك عمل عدة أشياء مفيدة للقوائم:

      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.

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_colors;
      my @colors = values %fruit_colors;

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.

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 $a = "foo";
  if ($some_condition) {
      my $b = "bar";
      print $a;           # prints "foo"
      print $b;           # prints "bar"
  }
  print $a;               # prints "foo"
  print $b;               # prints nothing; $b 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 $b would cause a compile-time error and prevent you from running the program. Using strict is highly recommended.

Perl has most of the usual conditional and looping constructs except for case/switch (but if you really want it, there is a Switch module in Perl 5.8 and newer, and on CPAN. See the section on modules, below, for more information about modules and CPAN).

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 (!CIcondition).
  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";
      }

      # you dont have to use the default $_ either...
      foreach my $key (keys %hash) {
          print "The value of $key is $hash{$key}\n";
      }

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

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 CIfunctionname.

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

      +   جمع
      -   طرح
      *   ضرب
      /   قسمة

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

      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).

      &&  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.)

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

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";

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(INFILE,  "input.txt")   or die "Cant open input.txt: $!";
  open(OUTFILE, ">output.txt") or die "Cant open output.txt: $!";
  open(LOGFILE, ">>my.log")    or die "Cant open logfile: $!";

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  = <INFILE>;
  my @lines = <INFILE>;

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 (<INFILE>) {     # 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 OUTFILE $record;
  print LOGFILE $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 INFILE;

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:

      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).

      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.

  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}/         a 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;
      }

  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";
      }

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

Writing subroutines is easy:

  sub log {
      my $logmessage = shift;
      print LOGFILE $logmessage;
  }

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;
  }

For more information on writing subroutines, see perlsub.

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 perlboot, perltoot, perltooc 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.

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 CIModule::Name. Typically you will want to use CIModule::Name, 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.

Kirrily Skud Robert skud@cpan.org