Checking the Syntax - Page 3

Before we go on to more complicated patterns, let's just have a quick look at that syntax. As we noted previously, a lot of Perl's operations take $_ as a default argument, and regular expressions are one such operation. Since we have the text we want to test in $_, we don't need to use the =~ operator to 'bind' the pattern to another string. We could write the above even more simply:

$_ = "Nobody wants to hurt you... 'cept, I do
      hurt people sometimes, Case.";

if (/people/) {
print "Hooray! Found the word 'people'\n";}

Alternatively, we might want to test for the pattern not matching - the word not being found. Obviously, we could say unless (/people/), but if the text we're looking at isn't in $_, we may also use the negative form of that =~ operator, which is !~. For example:

#!/usr/bin/perl
# nomatch.plx
use warnings;
use strict;
my $gibson =
"Nobody wants to hurt you... 'cept,
	I do hurt people sometimes, Case.";
if ($gibson !~ /fish/) {
print "There are no fish in William Gibson.\n";
}

True to form, for cyberpunk books that don't regularly involve fish, we get the result.

>perl nomatch.plx

There are no fish in William Gibson.
>

Literal text is the simplest regular expression of all to look for, but we needn't look for just the one word - we could look for any particular phrase. However, we need to make sure that we exactly match all the characters: words (with correct capitalization), numbers, punctuation, and even whitespace:

#!/usr/bin/perl
# match2.plx
use warnings;
use strict;
$_ = "Nobody wants to hurt you... 'cept,
	I do hurt people sometimes, Case.";
if (/I do/) {
print "'I do' is in that string.\n";
}
if (/sometimes Case/) {
print "'sometimes Case' matched.\n";
}

Let's run this program and see what happens:

>perl match2.plx
'I do' is in that string.
>

The other string didn't match, even though those two words are there. This is because everything in a regular expression has to match the string, from start to finish: first "sometimes", then a space, then "Case". In $_, there was a comma before the space, so it didn't match exactly. Similarly, spaces inside the pattern are significant:

#!/usr/bin/perl
# match3.plx
use warnings;
use strict;
my $test1 = "The dog is in the kennel";
my $test2 = "The sheepdog is in the field";
if ($test1 =~ / dog/) {
print "This dog's at home.\n";
} 
if ($test2 =~ / dog/) {
print "This dog's at work.\n";
}

This will only find the first dog, as Perl was looking for a space followed by the three letters, 'dog':

>perl match3.plx
This dog's at home.
>

So, for the moment, it looks like we shall have to specify our patterns with absolute precision. As another example, look at this:

#!/usr/bin/perl
# match4.plx
use warnings;
use strict;


$_ = "Nobody wants to hurt you... 'cept, I do
	hurt people sometimes, Case.";
if (/case/) {
print "I guess it's just the way I'm made.\n";
} else {
print "Case? Where are you, Case?\n";
}

> perl match4.plx
Case? Where are you, Case?
>

Hmm, no match. Why not? Because we asked for a small 'c' when we had a big 'C' - regexps are (if you'll pardon the pun) case-sensitive. We can get around this by asking Perl to compare insensitively, and we do this by putting an 'i' (for 'insensitive') after the closing slash. If we alter the code above as follows:

if (/case/i) {

print "I guess it's just the way I'm made.\n";}
else { print "Case? Where are you, Case?\n";}

then we find him:

>perl match4.plx
I guess it's just the way I'm made.
>

This 'i ' is one of several modifiers that we can add to the end of the regular expression to change its behavior slightly. We'll see more of them later on.

Regular Expressions - Page 2
Beginning Perl
Interpolation - Page 4