There's another interesting method for speeding up regex searches as well - a trigram index[1]. This is what Google's code search used to do. A simple command-line version written in Go[2] was released for local use. There's even an ack replacement based upon it[3].

[1] http://swtch.com/~rsc/regexp/regexp4.html [2] http://code.google.com/p/codesearch/ [3] https://github.com/rliebling/fastrAck