Author: Jeffrey E. F. Friedl
WWW book information: http://www.oreilly.com/catalog/regex/
More book information: http://enterprise.ic.gc.ca/%7Ejfriedl/regex/
This book explains regular expressions in general, and then covers a number of different tools explaining specialized variations. The book focuses more on Perl, but Tcl is one of the tools covered in its own special section. And more...
There is a helpful errata section for this book on O'Reilly's site at: http://public.yahoo.com/~jfriedl/regex/errata.html