I believe (perhaps wrong again) that back references are not in posix (though many implementations have them) edit: I was wrong. back references are in posix.

I believe (perhaps wrong again) that back references are not in posix (though many implementations have them) edit: I was wrong. back references are in posix. https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap09.html https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1... so I guess not even all posix RE can be converted to finite state machines.