go-regexp
Pure-Go regular expressions with lookahead, lookbehind and backreferences — the constructs RE2 forbids.
go-regexp is a pure-Go, CGO_ENABLED=0 regular-expression engine of the Onigmo lineage with a standard-library-regexp-shaped API, adding lookahead, lookbehind, backreferences, atomic groups and subexpression calls that RE2 rejects at compile time. It matches on a backtracking VM and falls back to a linear-time lazy-NFA/DFA accelerator for the RE2-compatible subset, bounded by a step/time budget.