This package is extensively documented, and comes with a large test suite. From the comments in the package:
Regular expressions are a language for specifying text to ease the searching and manipulation of text. A complete discussion of regular expressions is beyond the scope of this document. See Jeffrey Friedl, Mastering Regular Expressions, by O'Reilly for a relatively complete. The regular expressions supported by this package are similar to those presently used in Perl 5.0 5 and Python, and are based upon Philip Hazel's excellent PCRE libraries (incorporated almost without change, subject to a free license described in Re aLicenseComment. Thanks are due to Markus Kohler and Stephen Pair for their assistance in the initial ports of early versions of the Plugin.