| From: | Mark Dilger <mark(dot)dilger(at)enterprisedb(dot)com> |
|---|---|
| To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Cc: | Andrew Dunstan <andrew(at)dunslane(dot)net>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>, Joel Jacobson <joel(at)compiler(dot)org> |
| Subject: | Re: Another regexp performance improvement: skip useless paren-captures |
| Date: | 2021-08-10 01:17:40 |
| Message-ID: | CD86BE1C-C5F9-4AC5-A932-B26CC5F1FE15@enterprisedb.com |
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| Lists: | pgsql-hackers |
> On Aug 9, 2021, at 6:11 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
> Hm. I'm not sure that this example proves anything about Perl's handling
> of the situation, since you didn't use a backref.
Well, this doesn't die either:
if ('foo' =~ m/((??{ die; })(.)(??{ die $1; })){0}((??{ die "got here"; })|\2)/)
{
print "matched\n";
}
The point is that the regex engine never walks the part of the pattern that {0} qualifies. I thought it was more clear in the prior example, because that example proves that the engine does get as far as capturing. This example also does that, and with a backref, because it dies with "got here".
—
Mark Dilger
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
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