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...
A second edition of this book is now available, and Tcl continues to be covered in it.
There is a helpful errata section for this book on O'Reilly's site at: http://public.yahoo.com/~jfriedl/regex/errata.html
The second edition [L1 ] praises Tcl's regular expression engine: Henry's Tcl regex engine is an important step forward.
Also, in Chapter 6: Crafting and Efficient Expression, Friedl says: Most of what this chapter talks about simply does not apply to Tcl. And, as the sidebar explains, Henry Spencer's Tcl regex is a hybrid DFA/NFA engine and it doesn't need most of the optimizations described in the chapter.