One of the DefinitivePerlBooks. This book is really not limited to Perl though. If you use regular expressions in any language or programming tool, you stand to profit from it.

''MasteringRegularExpressions'' by Jeffrey E. F. Friedl (aka TheOwlBook)

http://images.amazon.com/images/P/1565922573.01._PE_PI_SCMZZZZZZZ_.jpg

[ISBN 1-56592-257-3] The "Perl Way" is to slurp everything into a string, match a regular expression against it, and grab what matches. No other book, not even the Blue Camel, can tell you as much about how to make regular expressions reliable, efficient, and (gasp) even readable.

''MasteringRegularExpressions '' By Jeffrey E. F. Friedl, 2nd Edition 

http://images.amazon.com/images/P/0596002890.01._PE_PI_SCMZZZZZZZ_.jpg

[ISBN  0-596-00289-0] Quickly covers the basics of regularexpression syntax, then delves into the mechanics of expression-processing, common pitfalls, performance issues, and implementation-specific differences. Written in an engaging style and sprinkled with solutions to complex real-world problems, Mastering Regular Expressions offers a wealth information that you can put to immediate use.
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CategoryBook CategoryRegularExpressions