Regular expressions is the term used for a codified method of searching invented or defined by the American mathematician Stephen Kleene. The syntax (language format) described is compliant with extended regular expressions (EREs) defined in IEEE POSIX 1003.2 (Section 2.8). EREs are now commonly supported by Apache, PHP4, Javascript 1.3 , MS Visual Studio, MS Frontpage, most visual editors, vi, emac, the GNU family of tools (including grep, awk and sed) as well as many others. Extended Regular Expressions (EREs) will support Basic Regular Expressions (BREs are essentially a subset of EREs). Most applications, utilities and laguages that implement RE's extend the capabilities defined. The appropriate documentation should always be consulted.
Searchmonkey - power searching without the pain Searchmonkey bridges the gap between Beagle (simple interface, large search net) and find/grep (powerful searching, small search net).