[Bug libstdc++/85472] Regex match bug

[haberg-1 at telia dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85472

--- Comment #4 from Hans Åberg <haberg-1 at telia dot com> ---
(In reply to Tim Shen from comment #1)
> I know exactly how libc++ produces this result. It creates an empty
> match_result during each repetition ("*" operator). It's less confusing but
> much slower.

I wrote an NFA/DFA that computes all matches, it seems, in an efficient manner:
Action numbers are marked on the states of the sub-NFAs one is interested in.
While lexing a string, the partial reverse NFA is recorded, which is searched
when a final state is encountered. The string position action numbers, as
marked on the corresponding states, are recorded, and for each action number,
the contiguous string sequences are the possible submatches.