Horton

Re: Some regular-expression performance hacking

Chapman Flack <chap@anastigmatix.net>

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From: Chapman Flack <chap@anastigmatix.net>
To: Tom Lane <tgl@sss.pgh.pa.us>, Joel Jacobson <joel@compiler.org>
Cc: pgsql-hackers@lists.postgresql.org
Date: 2021-02-20T23:31:39Z
Lists: pgsql-hackers 
On 02/19/21 10:26, Tom Lane wrote:
>> "foo\nbar".match(/([\w\W]+)/)[1];
>> "foo
>> bar"
> 
> Oooh, that's very interesting.   I guess the advantage of that over using
> the 's' flag is that you can have different behaviors at different places
> in the same regex.


Perl, Python, and Java (at least) all have a common syntax for changing
flags locally in a non-capturing group, so you could just match (?s:.)
-- which I guess isn't any shorter than [\w\W] but makes the intent more
clear.

I see that JavaScript, for some reason, does not advertise that. We don't
either; we have (?:groups) without flags, and we have (?flags) but only
global at the start of the regex. Would it be worthwhile to jump on the
bandwagon and support local flags in groups?

We currently give 2201B: invalid regular expression: invalid embedded option
on an attempt to use the syntax, so implementing it couldn't break anything
someone is already doing.

Regards,
-Chap

Commits

  1. Suppress unnecessary regex subre nodes in a couple more cases.

  2. Improve memory management in regex compiler.

  3. Extend a test case a little

  4. Allow complemented character class escapes within regex brackets.

  5. Suppress compiler warning in new regex match-all detection code.

  6. Avoid generating extra subre tree nodes for capturing parentheses.

  7. Convert regex engine's subre tree from binary to N-ary style.

  8. Fix regex engine to suppress useless concatenation sub-REs.

  9. Recognize "match-all" NFAs within the regex engine.

  10. Invent "rainbow" arcs within the regex engine.

  11. Make some minor improvements in the regex code.

  12. Display the time when the process started waiting for the lock, in pg_locks, take 2

  13. README/C-comment: document GiST's NSN value

  14. doc: Mention NO DEPENDS ON EXTENSION in its supported ALTER commands