Libtre is an attempt to create a lightweight, robust, and efficient fully
POSIX compliant regexp matching library. There is still some work left, but
the results so far are promising.

At the core of Libtre is a new algorithm for regular expression matching with
submatch addressing. The algorithm uses linear worst-case time in the length
of the text being searched, and quadratic worst-case time in the length of the
used regular expression. In other words, the time complexity of the algorithm
is O(M2N), where M is the length of the regular expression and N is the length
of the text. The used space is also quadratic on the length of the regex, but
does not depend on the searched string. This quadratic behaviour occurs only
on pathological cases which are probably very rare in practice.

WWW: http://kouli.iki.fi/~vlaurika/tre/