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.