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Pattern Matching

One of Perl's real strengths is it's pattern matching capability. The most basic syntax is illustrated here:

$w = "The wings of desire bore Adrian ever higher over the darkling
abyss of life as a scullery lad.";

if ($w =~ /abyss/) {
   print "Too romantic! \n";
}

($w =~ /bore/) && print "Put some life into it! \n";

Regular Expressions

The object between the slashes specifying the pattern to match is called a regular expression.

Standard characters like '1' and 'f' match themselves.
Special characters match other stuff, like \t for tab, \s for white space, \d matches numeric characters, . matches anything, etc.
'Wildcard' characters can be used as follows:
- /a+/ matches 1 or more a's
- /a*/ matches 0 or more a's
- /a?/ matches 0 or 1 a's
[ab] matches a single 'a', or a single 'b', etc
[^qw] matches anything except a single 'q' or a single 'w'

Here are a few simple examples illustrating regular expressions.

"This is a test" =~ /T..s/;    # matches 'This'

"This is a test" =~ /T.*s/;    # matches 'This is a tes'

"This is a test" =~ /s[^\s]/   # matches 'st'

Substitution

Instead of simple matching, Perl allows for substitution as well. The basic syntax is:

$x = s/reg expr/thing to substitute/

This is particularly useful when used in conjunction with parentheses. If you include a pair of parentheses in a regular expression, the sub-expression that matches the regular expression they enclosed is stored in a variable called $1. If you have two pairs, the matches are stored in $1 and $2, etc.

$w = "This is a mistake dyslexic";

$w =~ s/(mistake) (dyslexic)/$2 $1 \n/;

print $w;

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Created: May 07 1996 --- Last modified: May 29 1996