handle lookahead and lookbehind
handle lookahead and lookbehind

Ignore-this: 88b17fc7e0ca10f0e9535816ee9b4154

darcs-hash:20100120002443-c41ad-26a92c2c02bfbb97ad32793f805bd6baca2963a0

vbmithr/ocaml-text,vbmithr/ocaml-text