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, 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 A C<L<use v5.35|perlfunc/use VERSION>> (or higher) declaration will
100 enable both C<strict> and C<warnings>:
105 =head2 Basic syntax overview
107 A Perl script or program consists of one or more statements. These
108 statements are simply written in the script in a straightforward
109 fashion. There is no need to have a C<main()> function or anything of
112 Perl statements end in a semi-colon:
114 print "Hello, world";
116 Comments start with a hash symbol and run to the end of the line
120 Whitespace is irrelevant:
126 ... except inside quoted strings:
128 # this would print with a linebreak in the middle
132 Double quotes or single quotes may be used around literal strings:
134 print "Hello, world";
135 print 'Hello, world';
137 However, only double quotes "interpolate" variables and special
138 characters such as newlines (C<\n>):
140 print "Hello, $name\n"; # works fine
141 print 'Hello, $name\n'; # prints $name\n literally
143 Numbers don't need quotes around them:
147 You can use parentheses for functions' arguments or omit them
148 according to your personal taste. They are only required
149 occasionally to clarify issues of precedence.
151 print("Hello, world\n");
152 print "Hello, world\n";
154 More detailed information about Perl syntax can be found in L<perlsyn>.
156 =head2 Perl variable types
158 Perl has three main variable types: scalars, arrays, and hashes.
164 A scalar represents a single value:
166 my $animal = "camel";
169 Scalar values can be strings, integers or floating point numbers, and Perl
170 will automatically convert between them as required. You have to declare
171 them using the C<my> keyword the first time you use them. (This is one of the
172 requirements of C<use strict;>.)
174 Scalar values can be used in various ways:
177 print "The animal is $animal\n";
178 print "The square of $answer is ", $answer * $answer, "\n";
180 Perl defines a number of special scalars with short names, often single
181 punctuation marks or digits. These variables are used for all
182 kinds of purposes, and are documented in L<perlvar>. The only one you
183 need to know about for now is C<$_> which is the "default variable".
184 It's used as the default argument to a number of functions in Perl, and
185 it's set implicitly by certain looping constructs.
187 print; # prints contents of $_ by default
191 An array represents a list of values:
193 my @animals = ("camel", "llama", "owl");
194 my @numbers = (23, 42, 69);
195 my @mixed = ("camel", 42, 1.23);
197 Arrays are zero-indexed. Here's how you get at elements in an array:
199 print $animals[0]; # prints "camel"
200 print $animals[1]; # prints "llama"
202 The special variable C<$#array> tells you the index of the last element
205 print $mixed[$#mixed]; # last element, prints 1.23
207 You might be tempted to use C<$#array + 1> to tell you how many items there
208 are in an array. Don't bother. As it happens, using C<@array> where Perl
209 expects to find a scalar value ("in scalar context") will give you the number
210 of elements in the array:
212 if (@animals < 5) { ... }
214 The elements we're getting from the array start with a C<$> because
215 we're getting just a single value out of the array; you ask for a scalar,
218 To get multiple values from an array:
220 @animals[0,1]; # gives ("camel", "llama");
221 @animals[0..2]; # gives ("camel", "llama", "owl");
222 @animals[1..$#animals]; # gives all except the first element
224 This is called an "array slice".
226 You can do various useful things to lists:
228 my @sorted = sort @animals;
229 my @backwards = reverse @numbers;
231 There are a couple of special arrays too, such as C<@ARGV> (the command
232 line arguments to your script) and C<@_> (the arguments passed to a
233 subroutine). These are documented in L<perlvar>.
237 A hash represents a set of key/value pairs:
239 my %fruit_color = ("apple", "red", "banana", "yellow");
241 You can use whitespace and the C<< => >> operator to lay them out more
249 To get at hash elements:
251 $fruit_color{"apple"}; # gives "red"
253 You can get at lists of keys and values with C<keys()> and
256 my @fruits = keys %fruit_color;
257 my @colors = values %fruit_color;
259 Hashes have no particular internal order, though you can sort the keys
260 and loop through them.
262 Just like special scalars and arrays, there are also special hashes.
263 The most well known of these is C<%ENV> which contains environment
264 variables. Read all about it (and other special variables) in
269 Scalars, arrays and hashes are documented more fully in L<perldata>.
271 More complex data types can be constructed using references, which allow
272 you to build lists and hashes within lists and hashes.
274 A reference is a scalar value and can refer to any other Perl data
275 type. So by storing a reference as the value of an array or hash
276 element, you can easily create lists and hashes within lists and
277 hashes. The following example shows a 2 level hash of hash
278 structure using anonymous hash references.
282 description => "single item",
286 description => "ordered list of items",
290 description => "key/value pairs",
295 print "Scalars begin with a $variables->{'scalar'}->{'sigil'}\n";
297 Exhaustive information on the topic of references can be found in
298 L<perlreftut>, L<perllol>, L<perlref> and L<perldsc>.
300 =head2 Variable scoping
302 Throughout the previous section all the examples have used the syntax:
306 The C<my> is actually not required; you could just use:
310 However, the above usage will create global variables throughout your
311 program, which is bad programming practice. C<my> creates lexically
312 scoped variables instead. The variables are scoped to the block
313 (i.e. a bunch of statements surrounded by curly-braces) in which they
317 my $some_condition = 1;
318 if ($some_condition) {
320 print $x; # prints "foo"
321 print $y; # prints "bar"
323 print $x; # prints "foo"
324 print $y; # prints nothing; $y has fallen out of scope
326 Using C<my> in combination with a C<use strict;> at the top of
327 your Perl scripts means that the interpreter will pick up certain common
328 programming errors. For instance, in the example above, the final
329 C<print $y> would cause a compile-time error and prevent you from
330 running the program. Using C<strict> is highly recommended.
332 =head2 Conditional and looping constructs
334 Perl has most of the usual conditional and looping constructs.
336 The conditions can be any Perl expression. See the list of operators in
337 the next section for information on comparison and boolean logic operators,
338 which are commonly used in conditional statements.
346 } elsif ( other condition ) {
352 There's also a negated version of it:
354 unless ( condition ) {
358 This is provided as a more readable version of C<if (!I<condition>)>.
360 Note that the braces are required in Perl, even if you've only got one
361 line in the block. However, there is a clever way of making your one-line
362 conditional blocks more English like:
364 # the traditional way
369 # the Perlish post-condition way
370 print "Yow!" if $zippy;
371 print "We have no bananas" unless $bananas;
375 while ( condition ) {
379 There's also a negated version, for the same reason we have C<unless>:
381 until ( condition ) {
385 You can also use C<while> in a post-condition:
387 print "LA LA LA\n" while 1; # loops forever
393 for ($i = 0; $i <= $max; $i++) {
397 The C style for loop is rarely needed in Perl since Perl provides
398 the more friendly list scanning C<foreach> loop.
403 print "This element is $_\n";
406 print $list[$_] foreach 0 .. $max;
408 # you don't have to use the default $_ either...
409 foreach my $key (keys %hash) {
410 print "The value of $key is $hash{$key}\n";
413 The C<foreach> keyword is actually a synonym for the C<for>
414 keyword. See C<L<perlsyn/"Foreach Loops">>.
418 For more detail on looping constructs (and some that weren't mentioned in
419 this overview) see L<perlsyn>.
421 =head2 Builtin operators and functions
423 Perl comes with a wide selection of builtin functions. Some of the ones
424 we've already seen include C<print>, C<sort> and C<reverse>. A list of
425 them is given at the start of L<perlfunc> and you can easily read
426 about any given function by using C<perldoc -f I<functionname>>.
428 Perl operators are documented in full in L<perlop>, but here are a few
429 of the most common ones:
440 =item Numeric comparison
446 <= less than or equal
447 >= greater than or equal
449 =item String comparison
455 le less than or equal
456 ge greater than or equal
458 (Why do we have separate numeric and string comparisons? Because we don't
459 have special variable types, and Perl needs to know whether to sort
460 numerically (where 99 is less than 100) or alphabetically (where 100 comes
469 (C<and>, C<or> and C<not> aren't just in the above table as descriptions
470 of the operators. They're also supported as operators in their own
471 right. They're more readable than the C-style operators, but have
472 different precedence to C<&&> and friends. Check L<perlop> for more
478 . string concatenation
479 x string multiplication (repeats strings)
480 .. range operator (creates a list of numbers or strings)
484 Many operators can be combined with a C<=> as follows:
486 $a += 1; # same as $a = $a + 1
487 $a -= 1; # same as $a = $a - 1
488 $a .= "\n"; # same as $a = $a . "\n";
492 You can open a file for input or output using the C<open()> function.
493 It's documented in extravagant detail in L<perlfunc> and L<perlopentut>,
496 open(my $in, "<", "input.txt") or die "Can't open input.txt: $!";
497 open(my $out, ">", "output.txt") or die "Can't open output.txt: $!";
498 open(my $log, ">>", "my.log") or die "Can't open my.log: $!";
500 You can read from an open filehandle using the C<< <> >> operator. In
501 scalar context it reads a single line from the filehandle, and in list
502 context it reads the whole file in, assigning each line to an element of
508 Reading in the whole file at one time is called slurping. It can
509 be useful but it may be a memory hog. Most text file processing
510 can be done a line at a time with Perl's looping constructs.
512 The C<< <> >> operator is most often seen in a C<while> loop:
514 while (<$in>) { # assigns each line in turn to $_
515 print "Just read in this line: $_";
518 We've already seen how to print to standard output using C<print()>.
519 However, C<print()> can also take an optional first argument specifying
520 which filehandle to print to:
522 print STDERR "This is your final warning.\n";
524 print $log $logmessage;
526 When you're done with your filehandles, you should C<close()> them
527 (though to be honest, Perl will clean up after you if you forget):
529 close $in or die "$in: $!";
531 =head2 Regular expressions
533 Perl's regular expression support is both broad and deep, and is the
534 subject of lengthy documentation in L<perlrequick>, L<perlretut>, and
535 elsewhere. However, in short:
539 =item Simple matching
541 if (/foo/) { ... } # true if $_ contains "foo"
542 if ($a =~ /foo/) { ... } # true if $a contains "foo"
544 The C<//> matching operator is documented in L<perlop>. It operates on
545 C<$_> by default, or can be bound to another variable using the C<=~>
546 binding operator (also documented in L<perlop>).
548 =item Simple substitution
550 s/foo/bar/; # replaces foo with bar in $_
551 $a =~ s/foo/bar/; # replaces foo with bar in $a
552 $a =~ s/foo/bar/g; # replaces ALL INSTANCES of foo with bar
555 The C<s///> substitution operator is documented in L<perlop>.
557 =item More complex regular expressions
559 You don't just have to match on fixed strings. In fact, you can match
560 on just about anything you could dream of by using more complex regular
561 expressions. These are documented at great length in L<perlre>, but for
562 the meantime, here's a quick cheat sheet:
565 \s a whitespace character (space, tab, newline,
567 \S non-whitespace character
570 \w a word character (a-z, A-Z, 0-9, _)
571 \W a non-word character
572 [aeiou] matches a single character in the given set
573 [^aeiou] matches a single character outside the given
575 (foo|bar|baz) matches any of the alternatives specified
580 Quantifiers can be used to specify how many of the previous thing you
581 want to match on, where "thing" means either a literal character, one
582 of the metacharacters listed above, or a group of characters or
583 metacharacters in parentheses.
585 * zero or more of the previous thing
586 + one or more of the previous thing
587 ? zero or one of the previous thing
588 {3} matches exactly 3 of the previous thing
589 {3,6} matches between 3 and 6 of the previous thing
590 {3,} matches 3 or more of the previous thing
594 /^\d+/ string starts with one or more digits
595 /^$/ nothing in the string (start and end are
597 /(\d\s){3}/ three digits, each followed by a whitespace
598 character (eg "3 4 5 ")
599 /(a.)+/ matches a string in which every odd-numbered
600 letter is a (eg "abacadaf")
602 # This loop reads from STDIN, and prints non-blank lines:
608 =item Parentheses for capturing
610 As well as grouping, parentheses serve a second purpose. They can be
611 used to capture the results of parts of the regexp match for later use.
612 The results end up in C<$1>, C<$2> and so on.
614 # a cheap and nasty way to break an email address up into parts
616 if ($email =~ /([^@]+)@(.+)/) {
617 print "Username is $1\n";
618 print "Hostname is $2\n";
621 =item Other regexp features
623 Perl regexps also support backreferences, lookaheads, and all kinds of
624 other complex details. Read all about them in L<perlrequick>,
625 L<perlretut>, and L<perlre>.
629 =head2 Writing subroutines
631 Writing subroutines is easy:
634 my $logmessage = shift;
635 open my $logfile, ">>", "my.log" or die "Could not open my.log: $!";
636 print $logfile $logmessage;
639 Now we can use the subroutine just as any other built-in function:
641 logger("We have a logger subroutine!");
643 What's that C<shift>? Well, the arguments to a subroutine are available
644 to us as a special array called C<@_> (see L<perlvar> for more on that).
645 The default argument to the C<shift> function just happens to be C<@_>.
646 So C<my $logmessage = shift;> shifts the first item off the list of
647 arguments and assigns it to C<$logmessage>.
649 We can manipulate C<@_> in other ways too:
651 my ($logmessage, $priority) = @_; # common
652 my $logmessage = $_[0]; # uncommon, and ugly
654 Subroutines can also return values:
658 my $result = $num * $num;
666 For more information on writing subroutines, see L<perlsub>.
670 OO Perl is relatively simple and is implemented using references which
671 know what sort of object they are based on Perl's concept of packages.
672 However, OO Perl is largely beyond the scope of this document.
673 Read L<perlootut> and L<perlobj>.
675 As a beginning Perl programmer, your most common use of OO Perl will be
676 in using third-party modules, which are documented below.
678 =head2 Using Perl modules
680 Perl modules provide a range of features to help you avoid reinventing
681 the wheel, and can be downloaded from CPAN ( L<http://www.cpan.org/> ). A
682 number of popular modules are included with the Perl distribution
685 Categories of modules range from text manipulation to network protocols
686 to database integration to graphics. A categorized list of modules is
687 also available from CPAN.
689 To learn how to install modules you download from CPAN, read
692 To learn how to use a particular module, use C<perldoc I<Module::Name>>.
693 Typically you will want to C<use I<Module::Name>>, which will then give
694 you access to exported functions or an OO interface to the module.
696 L<perlfaq> contains questions and answers related to many common
697 tasks, and often provides suggestions for good CPAN modules to use.
699 L<perlmod> describes Perl modules in general. L<perlmodlib> lists the
700 modules which came with your Perl installation.
702 If you feel the urge to write Perl modules, L<perlnewmod> will give you
707 Kirrily "Skud" Robert <skud@cpan.org>