A Functional Program for Regular Expressions Matching
Regular expressions  and tools to handle them, especially tools for regular expression matching—an early one is described in the seminal paper  by Ken Thompson—, are one of the major achievements of formal language and automata theory. Google counts 303,000 results for “regular expressions matching” (May 4, 2011); there are numerous command line tools for working with regular expressions such as grep; Google released a regular expression C++ library not long ago ; almost every programming language provides support for regular expressions; and even the text editor I am using to produce the source code of this LaTeX document has an extensive regular expression library.
- 3.Google: Re2: a principled approach to regular expression matching (March 11, 2010), http://google-opensource.blogspot.com/2010/03/re2-principled-approach-to-regular.html (press release)
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- 6.xkcd.com: Regular expressions, http://xkcd.com/208/