Regular Expressions in Perl

Figure 26-31

How It Works

The key part of xModifier.pl is how the content of the m// operator is laid out in the code. Notice in the last of the following lines that the x modifier is specified. That means unescaped whitespace inside the paired forward slashes of m// is ignored. Also, any characters from # to the end of a line are treated as comments.

This allows the pattern to be spread over several lines, and on each line, a comment can be added that explains the purpose of the component of the regular expression pattern on that line:

if ($myTestString =~

m/\d{5} # Match five numeric digits

(-\d{4})? # Optionally match a hyphen followed by four numeric digits /x)

Escaping Metacharacters

When you want to literally match characters that are used as metacharacters, you must escape the metacharacter using a preceding backslash character.

The following table summarizes the escaping needed for commonly used metacharacters, assuming that paired forward slashes are used to delimit a regular expression pattern.

Escaped Metacharacter	Unescaped Metacharacter
\/ (backslash followed	/ (forward slash)
by a forward slash)
\?	?
\*	*
\+	+

In Perl, because you can specify the delimiters for a regular expression pattern, the use of escaping can vary depending on what delimiter you have specified, as you will see in the next example.

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Chapter 26

Try It Out

Using Escaped Metacharacters

1.Type the following code in your chosen text editor:

#!/usr/bin/perl -w use strict;

my $myTestString = “http://www.w3.org/”; print “The test string is ‘$myTestString’.\n”;

print “There is a match.\n\n” if ($myTestString =~ m/http:\/\/.*/); print “The test string hasn’t changed but the pattern has.\n”;

print “Also the delimiter character is now paired ‘!’ characters.\n”; print “There is a match.\n\n” if ($myTestString =~ m!http://!);

print “The test string hasn’t changed and the pattern is the original one.\n”; print “Also the delimiter character is still paired ‘!’ characters.\n”;

print “There is a match.\n\n” if ($myTestString =~ m!http:\/\/!);

2.

3.

Save the code as EscapedMetacharacters.pl.

Run the code, and inspect the results, as shown in Figure 26-32.

Figure 26-32

How It Works

The first line assigns a URL (for the World Wide Web Consortium) to the $myTestString variable. Notice that the URL contains forward slash characters, as many URLs do:

my $myTestString = “http://www.w3.org/”;

The test string is output for the user’s information:

print “The test string is ‘$myTestString’.\n”;

If there is a match in the test string for the specified pattern, a message is displayed. Notice how the pattern is constructed. Each forward-slash character is escaped by a preceding backslash character. If you try to run the code with the pattern http://.* but fail to escape the forward slashes, an error message will be displayed:

print “There is	a match.\n\n” if ($myTestString =~ m/http:\/\/.*/);
print “The test	string hasn’t changed but the pattern has.\n”;
print “Also the	delimiter character is now paired ‘!’ characters.\n”;
print “There is	a match.\n\n” if ($myTestString =~ m!http://!);
print “The test	string hasn’t changed and the pattern is the original one.\n”;
print “Also the	delimiter character is still paired ‘!’ characters.\n”;
print “There is	a match.\n\n” if ($myTestString =~ m!http:\/\/!);

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