Intermediate Perl, 2nd Edition

Chapter 9. Regular Expression References

Beginning with v5.5, we can compile regular expressions and keep references to them without using a match or substitution operator. We can do all of our regular expression handling and preparation before we actually want to use them. Since a regular expression reference is just a scalar like any other reference, we can store it in an array or hash, pass it as an argument, interpolate it into a string, or use it in the many other ways we can use a scalar.

Before Regular Expression References

Most people usually see regular expressions as part of a match or substitution operator:

m/\bcoco.*/
s/\bcoconut\b/coconet/
split /coconut/, $string

The pattern, however, is separate from the operator, and the operator merely applies the pattern to a string.

We might already have an inkling of this since we can interpolate a regular expression into one of the operators:

my $pattern = 'coco.*';

if( m/$pattern/ ) {
  ...
}

In that case, $pattern is just a string like any other string, and it has no idea how we are going to use it. After the match operator interpolates the variable, it has to compile the resulting regular expression. The match has no idea ahead of time whether the pattern is valid, like it would in a literal pattern. Given an invalid pattern, the match might cause a fatal runtime error. To handle that, we can catch that at match time with an eval:

print 'Enter a pattern: ';
chomp( my $pattern = <STDIN> );

print "Enter some lines:\n";
while( <STDIN> ...