Regex Tester

Paste a pattern and some text to see every match and group. Go uses RE2 — no lookarounds or backreferences; it will tell you when you use them.

Notes

RE2 does not support (?=) (?! ) (?<=) (?<!) backreferences or recursion (PCRE does).
Matching is linear; there is no catastrophic backtracking, so (a+)+ cannot hang the service.
Named groups use (?P<name>...), not (?<name>...).