The main Perl regular expression tutorial can be found here, there's a much shorter summary of the main features here.

The main book on regular expressions is Mastering Regular Expressions, published by O'Reilly.

Boost.Regex forms the basis for the regular expression chapter of the Technical Report on C++ Library Extensions.

The Open Unix Specification contains a wealth of useful material, including the POSIX regular expression syntax.

The Pattern Matching Pointers site is a "must visit" resource for anyone interested in pattern matching.

Glimpse and Agrep, use a simplified regular expression syntax to achieve faster search times.

Udi Manber and Ricardo Baeza-Yates both have a selection of useful pattern matching papers available from their respective web sites.