efficient, principled regular expression library

 RE2 is a fast, safe, thread-friendly alternative to backtracking regular
 expression engines like those used in PCRE, Perl, and Python. It is a C++
 library.
 .
 RE2 uses automata theory to guarantee that regular expression searches run in
 time linear in the size of the input. RE2 implements memory limits, so that
 searches can be constrained to a fixed amount of memory. RE2 is engineered to
 use a small fixed C++ stack footprint no matter what inputs or regular
 expressions it must process; thus RE2 is useful in multithreaded environments
 where thread stacks cannot grow arbitrarily large.
 .
 On large inputs, RE2 is often much faster than backtracking engines; its use
 of automata theory lets it apply optimizations that the others cannot.