The Perl regular expression syntax includes a large number of special characters--meta characters--that make it possible to possible permutations of a pattern or the context in which a pattern may occur. The following list is not complete but only lists the most common ones.
As with variable interpolation in double-quoted strings, characters with special meanings also must be escaped in regular expressions if they are to be interpreted literally. Escaped characters are preceded by a backslash, \, character.
For example, a regular expression that matches a period in a string would look as follows:
$line =~ m/\./;
The period, ., has a special meaning in regular expressions. It matches ANY one character except a newline. For example, the following expression would match any string of 3 characters beginning with i and ending with d, like 'wild', 'period', or 'index':
$line =~ m/i.d/;
An upright slash, |, matches the regex before or after the slash. For example, the following expression will match either 'green' or 'blue':
$line =~ m/green|blue/;
Multiple alternation is possible, too. The following expression will match either 'yellow', 'green', or 'blue':
$line =~ m/yellow|green|blue/;
Quantifiers specify repetitions of the character or expression that precedes it:
* |
occurs 0 or more times |
+ |
occurs 1 or more times |
? |
occurs 0 or 1 time |
{n} |
occurs exactly n times |
{n,} |
occurs at least n times |
{n,m} |
occurs at least n times, but no more than m times |
Anchors don't match characters but places within a string:
^ |
matches at the beginning of a string or line |
$ |
matches at the end of a string or line |
\b |
matches at word boundary (between \w and \W) |
\B |
matches except at word boundary |