The latest version of the Perl Compatible Regular Expression handler (PCRE) which is used in MUSHclient comes with its own HTML documentation.
This is now available online at http://mushclient.com/pcre/. This "index" page describes a lot of things that as MUSHclient users you probably are not interested in (internal functions etc.) so you may prefer to go directly to http://mushclient.com/pcre/pcrepattern.html (Regular expressions supported by PCRE).