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.
Backtracking engines are typically full of features and convenient syntactic sugar but can be forced into taking exponential amounts of time on even small inputs. 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.