Working with RegExps - Page 13

Now that we've matched a string, what do we do with it? Well, sometimes it's just useful to know whether a string contains a given pattern or not. However, a lot of the time we're going to be doing search-and-replace operations on text. We'll explain how to do that here. We'll also cover some of the more advanced areas of dealing with regular expressions.

Substitution

The s is for 'substitute' - between the first two slashes, we put our regular expression as before. Before the final slash, we put our text replacement. Just as with matching, we can use the =~ operator to apply it to a certain string. If this is not given, it applies to the default variable $_ :

	#!/usr/bin/perl
	# subst1.plx
	use warnings;
	use strict;
	$_ = "Awake! Awake! Fear, Fire, Foes! Awake! Fire,
	 Foes! Awake!";# Tolkien, Lord of the Rings
	s/Foes/Flee/;
	print $_,"\n";

>perl subst1.plx
Awake! Awake! Fear, Fire, Flee! Awake! Fire, Foes! Awake!
>

Here we have substituted the first occurrence of 'Foes' with the word 'Flee'. Had we wanted to change every occurrence, we would have needed to use another modifier. Just as the /i modifier for matching case-insensitively, the /g modifier on a substitution acts globally:

	#!/usr/bin/perl# subst1.plxuse warnings;use strict;
	$_ = "Awake! Awake! Fear, Fire, Foes! Awake! Fire,
	 Foes! Awake!";#Tolkien, Lord of the Rings

	s/Foes/Flee/g;

	print $_,"\n";

> perl subst1.plx
Awake! Awake! Fear, Fire, Flee! Awake! Fire, Flee! Awake!
>

Like the left-hand side of the substitution, the right-hand side also works like a double-quoted string and is thus subject to variable interpolation. One useful thing, though, is that we can use the backreference variables we collected during the match on the right hand side. So, for instance, to swap the first two words in a string, we would say something like this:

	#!/usr/bin/perl
	# subst2.plx
	use warnings;
	use strict;
	$_ = "there are two major products that come out
	 of Berkeley: LSD and UNIX";
	# Jeremy Anderson
	s/(\w+)\s+(\w+)/$2 $1/;
	print $_, "?\n";

>perl subst2.plx
are there two major products that come out of Berkeley:
LSD and UNIX?
>

What would happen if we tried doing that globally? Well, let's do it and see:

	#!/usr/bin/perl# subst2.plxuse warnings;use strict;
	$_ = "there are two major products that come out
	 of Berkeley: LSD andUNIX ";# Jeremy Anderson

	s/(\w+)\s+(\w+)/$2 $1/g;

	print $_, "?\n";

>perl subst2.plx
are there major two that products out come Berkeley
 of: and LSD UNIX?
>

Here, every word in a pair is swapped with its neighbor. When processing a global match, Perl always starts where the previous match left off.

How the Engine Works - Page 12
Beginning Perl
Changing Delimiters - Page 14