Regular Expression Using Perl

Regular Expression Using Perl : Perl is the language that is the most famous for its use of regular expression for good reasons.

We use the =~ operator to denote a match or an assignment depending upon the context. The use of !~ is to reverse the sense of the match.

There are basically two regex operators in perl:

• Matching: m//
• Substitution: s///

The purpose of the // is to enclose the regex. However, any other delimiters like {}</codmy ($hours, $minutes, $seconds) = ($time =~ m/(\d+):(\d+):(\d+)/); e>, "", etc could be used.

Matching

To use the matching operator, we simply check both sides using the =~ and m// operator.

The following sets $true to 1 if and only if $foo matches the regular expression foo:

$true = ($foo =~ m/foo/);

It is not difficult to see that just the opposite is achieved with !~:

$false = ($foo !~ m/foo/);

Capturing

As promised, the () could be used for capturing parts of the regexes. When the pattern inside a parentheses match, they go into special variables like $1, $2, etc in that order.

Example:

Here’s how one would extract the hours, minutes, seconds from a time string:

if ($time =~ /(\d\d):(\d\d):(\d\d)/) { # match hh:mm:ss format
 $hours = $1;
 $minutes = $2;
 $seconds = $3;
}

In list context, the list ($1, $2, $3, .. ) would be returned.

my ($hours, $minutes, $seconds) = ($time =~ m/(\d+):(\d+):(\d+)/);

Substitution

This is our favorite search and replace feature. Almost the same syntax rules apply here except that there is an extra clause between the second // that tells us what to match with.

$x = "Time to feed the cat!";
$x =~ s/cat/hacker/; # $x contains "Time to feed the hacker!"
if ($x =~ s/^(Time.*hacker)!$/$1 now!/) {
 $more_insistent = 1;
}
$y = "'quoted words'";
$y =~ s/^'(.*)'$/$1/; # strip single quotes,
# $y contains "quoted words"

Modifiers

Modifiers could be appended to the end of the regex operation expression to modify their matching behavior.

Here is a list of some important modifiers:

Here’s how one might want to use the g modifier:

$x = "I batted 4 for 4";
$x =~ s/4/four/; # doesn't do it all:
# $x contains "I batted four for 4"

$x = "I batted 4 for 4";
$x =~ s/4/four/g; # does it all:
# $x contains "I batted four for four"