This page is about other sources of information on regular expressions.
Absolutely the best book on regular expressions (regexps) is Jeffrey E. F. Friedl’s O’Reilly-published book, Mastering Regular Expressions, (official errata page) (official homepage first edition January 1997 I.S.B.N. 1565922573, official homepage second edition I.S.B.N. 0596002890). It is mostly about regular expressions in general, but includes chapters on specific tools.
The first edition had a Chapter 6 with quite a lot on GNU Emacs specific information, including differences between Emacs regexps and those in other tools. The second edition gives an example of an elisp function featuring a call to
re-search-forward, mentions syntax classes like
\Sw, and considers GNU Emacs 21.2.1 in tools comparison tables, all scattered at relevant places in the text. (The 2nd edition devotes specific chapters to Perl, Java, and .NET, that have been particularly active on regular expressions during 2002.)