Perl for Web Site Management

Taking It for a Spin

That’s a lot of information to digest at one sitting. Let’s put some of this into practice to help you get a better grasp of it. For this test drive, we’ll be using the script shown in Example 7-1.

Example 7-1. A simple script to demonstrate regular expression behavior

#!/usr/bin/perl -w

# regex.plx

# test regular expression behavior

$string = 'Walnuts are very nutritious.';

if ($string =~ /Walnuts/) {
    print "Match!\n";
} else {
    print "No match!\n";
}

Running this script should yield the following output:

[jbc@andros regex]$ regex.plx
Match!

This shouldn’t be terribly surprising. We’re looking for the literal string Walnuts, which occurs inside the string, so we have a match.

Let’s try out that fancy word-boundary backslash sequence \b. We can do that by modifying the line containing the regular expression in regex.plx to look like this:

if ($string =~ /Wal\bnuts/) {

Now when we run the script we should get the following:

[jbc@andros regex]$ regex.plx
No match!

The \b makes it so that the expression can no longer match, since it doesn’t have a word boundary between Wal and nuts.

Now change that \b to a \B (which says there isn’t a word boundary at that location):

if ($string =~ /Wal\Bnuts/) {

When we run the script it should give us:

[jbc@andros regex]$ regex.plx
Match!

Notice how the \B sequence doesn’t actually take up any space in the match. That is, /Wal\Bnuts/ can still match the string Walnuts, even though it has that \B sequence stuck in the middle of it. The ...