To run a Perl program from the Unix command line:
- perl progname.pl
+ perl progname.pl
Alternatively, put this as the first line of your script:
- #!/usr/bin/env perl
+ #!/usr/bin/env perl
... and run the script as F</path/to/script.pl>. Of course, it'll need
to be executable first, so C<chmod 755 script.pl> (under Unix).
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;
+ #!/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
Perl statements end in a semi-colon:
- print "Hello, world";
+ print "Hello, world";
Comments start with a hash symbol and run to the end of the line
- # This is a comment
+ # This is a comment
Whitespace is irrelevant:
- print
- "Hello, world"
- ;
+ print
+ "Hello, world"
+ ;
... except inside quoted strings:
- # this would print with a linebreak in the middle
- print "Hello
- world";
+ # 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';
+ print "Hello, world";
+ print 'Hello, world';
However, only double quotes "interpolate" variables and special
characters such as newlines (C<\n>):
- print "Hello, $name\n"; # works fine
- print 'Hello, $name\n'; # prints $name\n literally
+ print "Hello, $name\n"; # works fine
+ print 'Hello, $name\n'; # prints $name\n literally
Numbers don't need quotes around them:
- print 42;
+ 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";
+ print("Hello, world\n");
+ print "Hello, world\n";
More detailed information about Perl syntax can be found in L<perlsyn>.
A scalar represents a single value:
- my $animal = "camel";
- my $answer = 42;
+ 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
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";
+ 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
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
+ print; # prints contents of $_ by default
=item Arrays
An array represents a list of values:
- my @animals = ("camel", "llama", "owl");
- my @numbers = (23, 42, 69);
- my @mixed = ("camel", 42, 1.23);
+ 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"
+ print $animals[0]; # prints "camel"
+ print $animals[1]; # prints "llama"
The special variable C<$#array> tells you the index of the last element
of an array:
- print $mixed[$#mixed]; # last element, prints 1.23
+ print $mixed[$#mixed]; # last element, prints 1.23
You might be tempted to use C<$#array + 1> to tell you how many items there
are in an array. Don't bother. As it happens, using C<@array> where Perl
expects to find a scalar value ("in scalar context") will give you the number
of elements in the array:
- if (@animals < 5) { ... }
+ if (@animals < 5) { ... }
The elements we're getting from the array start with a C<$> because
we're getting just a single value out of the array; you ask for a scalar,
To get multiple values from an array:
- @animals[0,1]; # gives ("camel", "llama");
- @animals[0..2]; # gives ("camel", "llama", "owl");
- @animals[1..$#animals]; # gives all except the first element
+ @animals[0,1]; # gives ("camel", "llama");
+ @animals[0..2]; # gives ("camel", "llama", "owl");
+ @animals[1..$#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;
+ my @sorted = sort @animals;
+ my @backwards = reverse @numbers;
There are a couple of special arrays too, such as C<@ARGV> (the command
line arguments to your script) and C<@_> (the arguments passed to a
A hash represents a set of key/value pairs:
- my %fruit_color = ("apple", "red", "banana", "yellow");
+ my %fruit_color = ("apple", "red", "banana", "yellow");
You can use whitespace and the C<< => >> operator to lay them out more
nicely:
- my %fruit_color = (
- apple => "red",
- banana => "yellow",
- );
+ my %fruit_color = (
+ apple => "red",
+ banana => "yellow",
+ );
To get at hash elements:
- $fruit_color{"apple"}; # gives "red"
+ $fruit_color{"apple"}; # gives "red"
You can get at lists of keys and values with C<keys()> and
C<values()>.
- my @fruits = keys %fruit_colors;
- my @colors = values %fruit_colors;
+ 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.
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";
+ 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
L<perlreftut>, L<perllol>, L<perlref> and L<perldsc>.
Throughout the previous section all the examples have used the syntax:
- my $var = "value";
+ my $var = "value";
The C<my> is actually not required; you could just use:
- $var = "value";
+ $var = "value";
However, the above usage will create global variables throughout your
program, which is bad programming practice. C<my> creates lexically
(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
+ 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; $y has fallen out of scope
Using C<my> in combination with a C<use strict;> at the top of
your Perl scripts means that the interpreter will pick up certain common
=item if
- if ( condition ) {
- ...
- } elsif ( other condition ) {
- ...
- } else {
- ...
- }
+ if ( condition ) {
+ ...
+ } elsif ( other condition ) {
+ ...
+ } else {
+ ...
+ }
There's also a negated version of it:
- unless ( condition ) {
- ...
- }
+ unless ( condition ) {
+ ...
+ }
This is provided as a more readable version of C<if (!I<condition>)>.
line in the block. However, there is a clever way of making your one-line
conditional blocks more English like:
- # the traditional way
- if ($zippy) {
- print "Yow!";
- }
+ # the traditional way
+ if ($zippy) {
+ print "Yow!";
+ }
- # the Perlish post-condition way
- print "Yow!" if $zippy;
- print "We have no bananas" unless $bananas;
+ # the Perlish post-condition way
+ print "Yow!" if $zippy;
+ print "We have no bananas" unless $bananas;
=item while
- while ( condition ) {
- ...
- }
+ while ( condition ) {
+ ...
+ }
There's also a negated version, for the same reason we have C<unless>:
- until ( condition ) {
- ...
- }
+ until ( condition ) {
+ ...
+ }
You can also use C<while> in a post-condition:
- print "LA LA LA\n" while 1; # loops forever
+ print "LA LA LA\n" while 1; # loops forever
=item for
Exactly like C:
- for ($i = 0; $i <= $max; $i++) {
- ...
- }
+ for ($i = 0; $i <= $max; $i++) {
+ ...
+ }
The C style for loop is rarely needed in Perl since Perl provides
the more friendly list scanning C<foreach> loop.
=item foreach
- foreach (@array) {
- print "This element is $_\n";
- }
+ foreach (@array) {
+ print "This element is $_\n";
+ }
- print $list[$_] foreach 0 .. $max;
+ 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";
- }
+ # you don't have to use the default $_ either...
+ foreach my $key (keys %hash) {
+ print "The value of $key is $hash{$key}\n";
+ }
The C<foreach> keyword is actually a synonym for the C<for>
keyword. See C<L<perlsyn/"Foreach Loops">>.
=item Arithmetic
- + addition
- - subtraction
- * multiplication
- / division
+ + addition
+ - subtraction
+ * multiplication
+ / division
=item Numeric comparison
- == equality
- != inequality
- < less than
- > greater than
- <= less than or equal
- >= greater than or equal
+ == equality
+ != inequality
+ < less than
+ > greater than
+ <= less than or equal
+ >= greater than or equal
=item String comparison
- eq equality
- ne inequality
- lt less than
- gt greater than
- le less than or equal
- ge 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
=item Boolean logic
- && and
- || or
- ! not
+ && and
+ || or
+ ! not
(C<and>, C<or> and C<not> aren't just in the above table as descriptions
of the operators. They're also supported as operators in their own
=item Miscellaneous
- = assignment
- . string concatenation
- x string multiplication
- .. range operator (creates a list of numbers)
+ = assignment
+ . string concatenation
+ x string multiplication
+ .. range operator (creates a list of numbers)
=back
Many operators can be combined with a C<=> as follows:
- $a += 1; # same as $a = $a + 1
- $a -= 1; # same as $a = $a - 1
- $a .= "\n"; # same as $a = $a . "\n";
+ $a += 1; # same as $a = $a + 1
+ $a -= 1; # same as $a = $a - 1
+ $a .= "\n"; # same as $a = $a . "\n";
=head2 Files and I/O
It's documented in extravagant detail in L<perlfunc> and L<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: $!";
+ 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 C<< <> >> 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>;
+ my $line = <$in>;
+ my @lines = <$in>;
Reading in the whole file at one time is called slurping. It can
be useful but it may be a memory hog. Most text file processing
The C<< <> >> operator is most often seen in a C<while> loop:
- while (<$in>) { # assigns each line in turn to $_
- print "Just read in this line: $_";
- }
+ while (<$in>) { # assigns each line in turn to $_
+ print "Just read in this line: $_";
+ }
We've already seen how to print to standard output using C<print()>.
However, C<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;
+ print STDERR "This is your final warning.\n";
+ print $out $record;
+ print $log $logmessage;
When you're done with your filehandles, you should C<close()> them
(though to be honest, Perl will clean up after you if you forget):
- close $in or die "$in: $!";
+ close $in or die "$in: $!";
=head2 Regular expressions
=item Simple matching
- if (/foo/) { ... } # true if $_ contains "foo"
- if ($a =~ /foo/) { ... } # true if $a contains "foo"
+ if (/foo/) { ... } # true if $_ contains "foo"
+ if ($a =~ /foo/) { ... } # true if $a contains "foo"
The C<//> matching operator is documented in L<perlop>. It operates on
C<$_> by default, or can be bound to another variable using the C<=~>
=item 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
+ 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 C<s///> substitution operator is documented in L<perlop>.
expressions. These are documented at great length in L<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
+ . 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
+ * 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
- /^$/ nothing in the string (start and end are adjacent)
- /(\d\s){3}/ a three digits, each followed by a whitespace
- character (eg "3 4 5 ")
- /(a.)+/ matches a string in which every odd-numbered letter
- is a (eg "abacadaf")
+ /^\d+/ string starts with one or more digits
+ /^$/ 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 /^$/;
- print;
- }
+ # This loop reads from STDIN, and prints non-blank lines:
+ while (<>) {
+ next if /^$/;
+ print;
+ }
=item Parentheses for capturing
used to capture the results of parts of the regexp match for later use.
The results end up in C<$1>, C<$2> and so on.
- # a cheap and nasty way to break an email address up into parts
+ # a cheap and nasty way to break an email address up into parts
- if ($email =~ /([^@]+)@(.+)/) {
- print "Username is $1\n";
- print "Hostname is $2\n";
- }
+ if ($email =~ /([^@]+)@(.+)/) {
+ print "Username is $1\n";
+ print "Hostname is $2\n";
+ }
=item Other regexp features
Writing subroutines is easy:
- sub logger {
- my $logmessage = shift;
- open my $logfile, ">>", "my.log" or die "Could not open my.log: $!";
- print $logfile $logmessage;
- }
+ sub logger {
+ my $logmessage = shift;
+ open my $logfile, ">>", "my.log" or die "Could not open my.log: $!";
+ print $logfile $logmessage;
+ }
Now we can use the subroutine just as any other built-in function:
- logger("We have a logger subroutine!");
+ logger("We have a logger subroutine!");
What's that C<shift>? Well, the arguments to a subroutine are available
to us as a special array called C<@_> (see L<perlvar> for more on that).
We can manipulate C<@_> in other ways too:
- my ($logmessage, $priority) = @_; # common
- my $logmessage = $_[0]; # uncommon, and ugly
+ my ($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;
- }
+ sub square {
+ my $num = shift;
+ my $result = $num * $num;
+ return $result;
+ }
Then use it like:
- $sq = square(8);
+ $sq = square(8);
For more information on writing subroutines, see L<perlsub>.
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