Re: [Emacs-diffs] master 938d252 4/4: Make regex matching reentrant; update syntax during match

["Daniel Colascione" <dancol@dancol.org>]

>>     (unfreeze_pattern, freeze_pattern): New functions.
>>     (compile_pattern): Return a regexp_cache pointer instead of the
>>     re_pattern_buffer, allowing callers to use `freeze_pattern' if
>>     needed.  Do not consider busy patterns as cache hit candidates;
>>     error if we run out of non-busy cache entries.
>
> IIRC the main/only reason why you can't use a compiled pattern in
> a reentrant way is because the \{N,M\} repetitions use a counter that's
> stored directly within the compiled pattern.
>
> But these are fairly rare.
>
> So we could easily change the code to add a boolean stating whether there
> is such a repetition-counter in the pattern, and if there isn't then
> "freeze" can just do nothing because we can freely use that pattern
> multiple times at the same time.

Good idea. My reading of the "smart jump" stuff in regex.c suggested that
we use the optimization for _all_ greedy Kleene star constructs though,
not just the bounded ones. Am I wrong?

But anyway, I think the regex code needs a major overhaul.  I was actually
thinking about forking and vendoring RE2. Granted, having done that, you'd
need a C++ compiler to build Emacs, but it's probably one of the better
actively-maintained DFA-based regex engines around.