NAME
perlintro -- a brief introduction and overview of Perl
DESCRIPTION
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.
What is Perl?
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.
Running Perl programs
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 "/path/to/script.pl". Of course, it’ll need
to be executable first, so "chmod 755 script.pl" (under Unix).
(This start line assumes you have the env 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 and Mac OS, read perlrun.
Safety net
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 -w) and let your code run.
To read more about them check their respective manual pages at strict
and warnings.
Basic syntax overview
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.
Perl variable types
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.
There is no need to pre-declare your variable types, but 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";
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
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_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.
Variable scoping
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 $y" would cause a compile-time error and prevent you from
running the program. Using "strict" is highly recommended.
Conditional and looping constructs
Perl has most of the usual conditional and looping constructs except
for case/switch (but if you really want it, there is a Switch module in
Perl 5.8 and newer, and on CPAN. See the section on modules, below, for
more information about modules and CPAN).
The conditions can be any Perl expression. See the list of operators
in the next section for information on comparison and boolean logic
operators, which are commonly used in conditional statements.
if
if ( condition ) {
...
} elsif ( other condition ) {
...
} else {
...
}
There’s also a negated version of it:
unless ( condition ) {
...
}
This is provided as a more readable version of "if (!condition)".
Note that the braces are required in Perl, even if you’ve only got
one line in the block. However, there is a clever way of making
your one-line conditional blocks more English like:
# the traditional way
if ($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";
}
For more detail on looping constructs (and some that weren’t mentioned
in this overview) see perlsyn.
Builtin operators and functions
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 functionname".
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
.. 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";
Files and I/O
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: $!";
Regular expressions
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
/^$/ nothing in the string (start and end are adjacent)
/(\d\s){3}/ a three digits, each followed by a whitespace
character (eg "3 4 5 ")
/(a.)+/ matches a string in which every odd-numbered letter
is a (eg "abacadaf")
# This loop reads from STDIN, and prints non-blank lines:
while (<>) {
next if /^$/;
print;
}
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.
Writing subroutines
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.
OO Perl
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.
Using Perl modules
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 Module::Name".
Typically you will want to "use Module::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.
AUTHOR
Kirrily "Skud" Robert <skud@cpan.org>