3 perlintro -- a brief introduction and overview of Perl
7 This document is intended to give you a quick overview of the Perl
8 programming language, along with pointers to further documentation. It
9 is intended as a "bootstrap" guide for those who are new to the
10 language, and provides just enough information for you to be able to
11 read other peoples' Perl and understand roughly what it's doing, or
12 write your own simple scripts.
14 This introductory document does not aim to be complete. It does not
15 even aim to be entirely accurate. In some cases perfection has been
16 sacrificed in the goal of getting the general idea across. You are
17 I<strongly> advised to follow this introduction with more information
18 from the full Perl manual, the table of contents to which can be found
21 Throughout this document you'll see references to other parts of the
22 Perl documentation. You can read that documentation using the C<perldoc>
23 command or whatever method you're using to read this document.
25 Throughout Perl's documentation, you'll find numerous examples intended
26 to help explain the discussed features. Please keep in mind that many
27 of them are code fragments rather than complete programs.
29 These examples often reflect the style and preference of the author of
30 that piece of the documentation, and may be briefer than a corresponding
31 line of code in a real program. Except where otherwise noted, you
32 should assume that C<use strict> and C<use warnings> statements
33 appear earlier in the "program", and that any variables used have
34 already been declared, even if those declarations have been omitted
35 to make the example easier to read.
37 Do note that the examples have been written by many different authors over
38 a period of several decades. Styles and techniques will therefore differ,
39 although some effort has been made to not vary styles too widely in the
40 same sections. Do not consider one style to be better than others - "There's
41 More Than One Way To Do It" is one of Perl's mottos. After all, in your
42 journey as a programmer, you are likely to encounter different styles.
46 Perl is a general-purpose programming language originally developed for
47 text manipulation and now used for a wide range of tasks including
48 system administration, web development, network programming, GUI
49 development, and more.
51 The language is intended to be practical (easy to use, efficient,
52 complete) rather than beautiful (tiny, elegant, minimal). Its major
53 features are that it's easy to use, supports both procedural and
54 object-oriented (OO) programming, has powerful built-in support for text
55 processing, and has one of the world's most impressive collections of
58 Different definitions of Perl are given in L<perl>, L<perlfaq1> and
59 no doubt other places. From this we can determine that Perl is different
60 things to different people, but that lots of people think it's at least
63 =head2 Running Perl programs
65 To run a Perl program from the Unix command line:
69 Alternatively, put this as the first line of your script:
73 ... and run the script as F</path/to/script.pl>. Of course, it'll need
74 to be executable first, so C<chmod 755 script.pl> (under Unix).
76 (This start line assumes you have the B<env> program. You can also put
77 directly the path to your perl executable, like in C<#!/usr/bin/perl>).
79 For more information, including instructions for other platforms such as
80 Windows and Mac OS, read L<perlrun>.
84 Perl by default is very forgiving. In order to make it more robust
85 it is recommended to start every program with the following lines:
91 The two additional lines request from perl to catch various common
92 problems in your code. They check different things so you need both. A
93 potential problem caught by C<use strict;> will cause your code to stop
94 immediately when it is encountered, while C<use warnings;> will merely
95 give a warning (like the command-line switch B<-w>) and let your code run.
96 To read more about them check their respective manual pages at L<strict>
99 =head2 Basic syntax overview
101 A Perl script or program consists of one or more statements. These
102 statements are simply written in the script in a straightforward
103 fashion. There is no need to have a C<main()> function or anything of
106 Perl statements end in a semi-colon:
108 print "Hello, world";
110 Comments start with a hash symbol and run to the end of the line
114 Whitespace is irrelevant:
120 ... except inside quoted strings:
122 # this would print with a linebreak in the middle
126 Double quotes or single quotes may be used around literal strings:
128 print "Hello, world";
129 print 'Hello, world';
131 However, only double quotes "interpolate" variables and special
132 characters such as newlines (C<\n>):
134 print "Hello, $name\n"; # works fine
135 print 'Hello, $name\n'; # prints $name\n literally
137 Numbers don't need quotes around them:
141 You can use parentheses for functions' arguments or omit them
142 according to your personal taste. They are only required
143 occasionally to clarify issues of precedence.
145 print("Hello, world\n");
146 print "Hello, world\n";
148 More detailed information about Perl syntax can be found in L<perlsyn>.
150 =head2 Perl variable types
152 Perl has three main variable types: scalars, arrays, and hashes.
158 A scalar represents a single value:
160 my $animal = "camel";
163 Scalar values can be strings, integers or floating point numbers, and Perl
164 will automatically convert between them as required. There is no need
165 to pre-declare your variable types, but you have to declare them using
166 the C<my> keyword the first time you use them. (This is one of the
167 requirements of C<use strict;>.)
169 Scalar values can be used in various ways:
172 print "The animal is $animal\n";
173 print "The square of $answer is ", $answer * $answer, "\n";
175 There are a number of "magic" scalars with names that look like
176 punctuation or line noise. These special variables are used for all
177 kinds of purposes, and are documented in L<perlvar>. The only one you
178 need to know about for now is C<$_> which is the "default variable".
179 It's used as the default argument to a number of functions in Perl, and
180 it's set implicitly by certain looping constructs.
182 print; # prints contents of $_ by default
186 An array represents a list of values:
188 my @animals = ("camel", "llama", "owl");
189 my @numbers = (23, 42, 69);
190 my @mixed = ("camel", 42, 1.23);
192 Arrays are zero-indexed. Here's how you get at elements in an array:
194 print $animals[0]; # prints "camel"
195 print $animals[1]; # prints "llama"
197 The special variable C<$#array> tells you the index of the last element
200 print $mixed[$#mixed]; # last element, prints 1.23
202 You might be tempted to use C<$#array + 1> to tell you how many items there
203 are in an array. Don't bother. As it happens, using C<@array> where Perl
204 expects to find a scalar value ("in scalar context") will give you the number
205 of elements in the array:
207 if (@animals < 5) { ... }
209 The elements we're getting from the array start with a C<$> because
210 we're getting just a single value out of the array; you ask for a scalar,
213 To get multiple values from an array:
215 @animals[0,1]; # gives ("camel", "llama");
216 @animals[0..2]; # gives ("camel", "llama", "owl");
217 @animals[1..$#animals]; # gives all except the first element
219 This is called an "array slice".
221 You can do various useful things to lists:
223 my @sorted = sort @animals;
224 my @backwards = reverse @numbers;
226 There are a couple of special arrays too, such as C<@ARGV> (the command
227 line arguments to your script) and C<@_> (the arguments passed to a
228 subroutine). These are documented in L<perlvar>.
232 A hash represents a set of key/value pairs:
234 my %fruit_color = ("apple", "red", "banana", "yellow");
236 You can use whitespace and the C<< => >> operator to lay them out more
244 To get at hash elements:
246 $fruit_color{"apple"}; # gives "red"
248 You can get at lists of keys and values with C<keys()> and
251 my @fruits = keys %fruit_colors;
252 my @colors = values %fruit_colors;
254 Hashes have no particular internal order, though you can sort the keys
255 and loop through them.
257 Just like special scalars and arrays, there are also special hashes.
258 The most well known of these is C<%ENV> which contains environment
259 variables. Read all about it (and other special variables) in
264 Scalars, arrays and hashes are documented more fully in L<perldata>.
266 More complex data types can be constructed using references, which allow
267 you to build lists and hashes within lists and hashes.
269 A reference is a scalar value and can refer to any other Perl data
270 type. So by storing a reference as the value of an array or hash
271 element, you can easily create lists and hashes within lists and
272 hashes. The following example shows a 2 level hash of hash
273 structure using anonymous hash references.
277 description => "single item",
281 description => "ordered list of items",
285 description => "key/value pairs",
290 print "Scalars begin with a $variables->{'scalar'}->{'sigil'}\n";
292 Exhaustive information on the topic of references can be found in
293 L<perlreftut>, L<perllol>, L<perlref> and L<perldsc>.
295 =head2 Variable scoping
297 Throughout the previous section all the examples have used the syntax:
301 The C<my> is actually not required; you could just use:
305 However, the above usage will create global variables throughout your
306 program, which is bad programming practice. C<my> creates lexically
307 scoped variables instead. The variables are scoped to the block
308 (i.e. a bunch of statements surrounded by curly-braces) in which they
312 my $some_condition = 1;
313 if ($some_condition) {
315 print $x; # prints "foo"
316 print $y; # prints "bar"
318 print $x; # prints "foo"
319 print $y; # prints nothing; $y has fallen out of scope
321 Using C<my> in combination with a C<use strict;> at the top of
322 your Perl scripts means that the interpreter will pick up certain common
323 programming errors. For instance, in the example above, the final
324 C<print $y> would cause a compile-time error and prevent you from
325 running the program. Using C<strict> is highly recommended.
327 =head2 Conditional and looping constructs
329 Perl has most of the usual conditional and looping constructs. As of Perl
330 5.10, it even has a case/switch statement (spelled C<given>/C<when>). See
331 L<perlsyn/"Switch Statements"> for more details.
333 The conditions can be any Perl expression. See the list of operators in
334 the next section for information on comparison and boolean logic operators,
335 which are commonly used in conditional statements.
343 } elsif ( other condition ) {
349 There's also a negated version of it:
351 unless ( condition ) {
355 This is provided as a more readable version of C<if (!I<condition>)>.
357 Note that the braces are required in Perl, even if you've only got one
358 line in the block. However, there is a clever way of making your one-line
359 conditional blocks more English like:
361 # the traditional way
366 # the Perlish post-condition way
367 print "Yow!" if $zippy;
368 print "We have no bananas" unless $bananas;
372 while ( condition ) {
376 There's also a negated version, for the same reason we have C<unless>:
378 until ( condition ) {
382 You can also use C<while> in a post-condition:
384 print "LA LA LA\n" while 1; # loops forever
390 for ($i = 0; $i <= $max; $i++) {
394 The C style for loop is rarely needed in Perl since Perl provides
395 the more friendly list scanning C<foreach> loop.
400 print "This element is $_\n";
403 print $list[$_] foreach 0 .. $max;
405 # you don't have to use the default $_ either...
406 foreach my $key (keys %hash) {
407 print "The value of $key is $hash{$key}\n";
410 The C<foreach> keyword is actually a synonym for the C<for>
411 keyword. See C<L<perlsyn/"Foreach Loops">>.
415 For more detail on looping constructs (and some that weren't mentioned in
416 this overview) see L<perlsyn>.
418 =head2 Builtin operators and functions
420 Perl comes with a wide selection of builtin functions. Some of the ones
421 we've already seen include C<print>, C<sort> and C<reverse>. A list of
422 them is given at the start of L<perlfunc> and you can easily read
423 about any given function by using C<perldoc -f I<functionname>>.
425 Perl operators are documented in full in L<perlop>, but here are a few
426 of the most common ones:
437 =item Numeric comparison
443 <= less than or equal
444 >= greater than or equal
446 =item String comparison
452 le less than or equal
453 ge greater than or equal
455 (Why do we have separate numeric and string comparisons? Because we don't
456 have special variable types, and Perl needs to know whether to sort
457 numerically (where 99 is less than 100) or alphabetically (where 100 comes
466 (C<and>, C<or> and C<not> aren't just in the above table as descriptions
467 of the operators. They're also supported as operators in their own
468 right. They're more readable than the C-style operators, but have
469 different precedence to C<&&> and friends. Check L<perlop> for more
475 . string concatenation
476 x string multiplication
477 .. range operator (creates a list of numbers)
481 Many operators can be combined with a C<=> as follows:
483 $a += 1; # same as $a = $a + 1
484 $a -= 1; # same as $a = $a - 1
485 $a .= "\n"; # same as $a = $a . "\n";
489 You can open a file for input or output using the C<open()> function.
490 It's documented in extravagant detail in L<perlfunc> and L<perlopentut>,
493 open(my $in, "<", "input.txt") or die "Can't open input.txt: $!";
494 open(my $out, ">", "output.txt") or die "Can't open output.txt: $!";
495 open(my $log, ">>", "my.log") or die "Can't open my.log: $!";
497 You can read from an open filehandle using the C<< <> >> operator. In
498 scalar context it reads a single line from the filehandle, and in list
499 context it reads the whole file in, assigning each line to an element of
505 Reading in the whole file at one time is called slurping. It can
506 be useful but it may be a memory hog. Most text file processing
507 can be done a line at a time with Perl's looping constructs.
509 The C<< <> >> operator is most often seen in a C<while> loop:
511 while (<$in>) { # assigns each line in turn to $_
512 print "Just read in this line: $_";
515 We've already seen how to print to standard output using C<print()>.
516 However, C<print()> can also take an optional first argument specifying
517 which filehandle to print to:
519 print STDERR "This is your final warning.\n";
521 print $log $logmessage;
523 When you're done with your filehandles, you should C<close()> them
524 (though to be honest, Perl will clean up after you if you forget):
526 close $in or die "$in: $!";
528 =head2 Regular expressions
530 Perl's regular expression support is both broad and deep, and is the
531 subject of lengthy documentation in L<perlrequick>, L<perlretut>, and
532 elsewhere. However, in short:
536 =item Simple matching
538 if (/foo/) { ... } # true if $_ contains "foo"
539 if ($a =~ /foo/) { ... } # true if $a contains "foo"
541 The C<//> matching operator is documented in L<perlop>. It operates on
542 C<$_> by default, or can be bound to another variable using the C<=~>
543 binding operator (also documented in L<perlop>).
545 =item Simple substitution
547 s/foo/bar/; # replaces foo with bar in $_
548 $a =~ s/foo/bar/; # replaces foo with bar in $a
549 $a =~ s/foo/bar/g; # replaces ALL INSTANCES of foo with bar
552 The C<s///> substitution operator is documented in L<perlop>.
554 =item More complex regular expressions
556 You don't just have to match on fixed strings. In fact, you can match
557 on just about anything you could dream of by using more complex regular
558 expressions. These are documented at great length in L<perlre>, but for
559 the meantime, here's a quick cheat sheet:
562 \s a whitespace character (space, tab, newline,
564 \S non-whitespace character
567 \w a word character (a-z, A-Z, 0-9, _)
568 \W a non-word character
569 [aeiou] matches a single character in the given set
570 [^aeiou] matches a single character outside the given
572 (foo|bar|baz) matches any of the alternatives specified
577 Quantifiers can be used to specify how many of the previous thing you
578 want to match on, where "thing" means either a literal character, one
579 of the metacharacters listed above, or a group of characters or
580 metacharacters in parentheses.
582 * zero or more of the previous thing
583 + one or more of the previous thing
584 ? zero or one of the previous thing
585 {3} matches exactly 3 of the previous thing
586 {3,6} matches between 3 and 6 of the previous thing
587 {3,} matches 3 or more of the previous thing
591 /^\d+/ string starts with one or more digits
592 /^$/ nothing in the string (start and end are
594 /(\d\s){3}/ three digits, each followed by a whitespace
595 character (eg "3 4 5 ")
596 /(a.)+/ matches a string in which every odd-numbered
597 letter is a (eg "abacadaf")
599 # This loop reads from STDIN, and prints non-blank lines:
605 =item Parentheses for capturing
607 As well as grouping, parentheses serve a second purpose. They can be
608 used to capture the results of parts of the regexp match for later use.
609 The results end up in C<$1>, C<$2> and so on.
611 # a cheap and nasty way to break an email address up into parts
613 if ($email =~ /([^@]+)@(.+)/) {
614 print "Username is $1\n";
615 print "Hostname is $2\n";
618 =item Other regexp features
620 Perl regexps also support backreferences, lookaheads, and all kinds of
621 other complex details. Read all about them in L<perlrequick>,
622 L<perlretut>, and L<perlre>.
626 =head2 Writing subroutines
628 Writing subroutines is easy:
631 my $logmessage = shift;
632 open my $logfile, ">>", "my.log" or die "Could not open my.log: $!";
633 print $logfile $logmessage;
636 Now we can use the subroutine just as any other built-in function:
638 logger("We have a logger subroutine!");
640 What's that C<shift>? Well, the arguments to a subroutine are available
641 to us as a special array called C<@_> (see L<perlvar> for more on that).
642 The default argument to the C<shift> function just happens to be C<@_>.
643 So C<my $logmessage = shift;> shifts the first item off the list of
644 arguments and assigns it to C<$logmessage>.
646 We can manipulate C<@_> in other ways too:
648 my ($logmessage, $priority) = @_; # common
649 my $logmessage = $_[0]; # uncommon, and ugly
651 Subroutines can also return values:
655 my $result = $num * $num;
663 For more information on writing subroutines, see L<perlsub>.
667 OO Perl is relatively simple and is implemented using references which
668 know what sort of object they are based on Perl's concept of packages.
669 However, OO Perl is largely beyond the scope of this document.
670 Read L<perlootut> and L<perlobj>.
672 As a beginning Perl programmer, your most common use of OO Perl will be
673 in using third-party modules, which are documented below.
675 =head2 Using Perl modules
677 Perl modules provide a range of features to help you avoid reinventing
678 the wheel, and can be downloaded from CPAN ( http://www.cpan.org/ ). A
679 number of popular modules are included with the Perl distribution
682 Categories of modules range from text manipulation to network protocols
683 to database integration to graphics. A categorized list of modules is
684 also available from CPAN.
686 To learn how to install modules you download from CPAN, read
689 To learn how to use a particular module, use C<perldoc I<Module::Name>>.
690 Typically you will want to C<use I<Module::Name>>, which will then give
691 you access to exported functions or an OO interface to the module.
693 L<perlfaq> contains questions and answers related to many common
694 tasks, and often provides suggestions for good CPAN modules to use.
696 L<perlmod> describes Perl modules in general. L<perlmodlib> lists the
697 modules which came with your Perl installation.
699 If you feel the urge to write Perl modules, L<perlnewmod> will give you
704 Kirrily "Skud" Robert <skud@cpan.org>