cfgrep is an attempt to produce a grep-like program that uses regular expressions extended to describe context-free grammars. The largest part of the project is the C++ library developed to process those expressions.
glark offers grep-like searching of text files, with very powerful, complex regular expressions (e.g., "/foo\w+/ and /bar[^\d]*baz$/ within 4 lines of each other"), as well as highlighting of matches.