re2
Alternative to backtracking regular expression engines
...In POSIX mode, RE2 accepts standard POSIX (egrep) syntax regular expressions. In Perl mode, RE2 accepts most Perl operators. The only excluded ones are those that require backtracking (and its potential for exponential runtime) to implement. These include backreferences (submatching is still okay) and generalized assertions. The Syntax page documents the supported Perl-mode syntax in detail.