Introduction

The syntax and semantics of the regular expressions
supported by PCRE are described below. Regular expressions are also
described in the Perl documentation and in a number of other books,
some of which have copious examples. Jeffrey Friedl’s “Mastering
Regular Expressions”, published by O’Reilly (ISBN 1-56592-257-3),
covers them in great detail. The description here is intended as
reference documentation.

A regular expression is a pattern that is matched
against a subject string from left to right. Most characters stand
for themselves in a pattern, and match the corresponding characters
in the subject. As a trivial example, the pattern The quick
brown fox matches a portion of a subject string that is
identical to itself.