Re: BUG #13538: REGEX non-greedy is working incorrectly (and also greedy matches fail if non-greedy is present)

On Tue, Aug 4, 2015 at 8:39 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> I wrote:
> > As David says, these examples appear to be following what's stated in
> >
> http://www.postgresql.org/docs/9.4/static/functions-matching.html#POSIX-MATCHING-RULES
> > The Spencer regex engine we use has a notion of greediness or
> > non-greediness of the entire regex, and further that that takes
> precedence
> > for determining the overall match length over greediness of individual
> > subexpressions. That behavior might be inconvenient for this particular
> > use-case, but that doesn't make it a bug.
>
> BTW, perhaps it would be worth adding an example to that section that
> shows how to control this behavior. The trick is obvious once you've seen
> it, but not so much otherwise: you add something to the start of the regex
> that establishes the overall greediness you want, but can never actually
> match any characters. "\0*" or "\0*?" will work fine in Postgres
> use-cases since there can never be a NUL character in the data.
>
> regression=# select regexp_matches('abc01234xyz', '(.*)(\d+)(.*)');
> regexp_matches
> -----------------
> {abc0123,4,xyz}
> (1 row)
>
> regression=# select regexp_matches('abc01234xyz', '(.*?)(\d+)(.*)');
> regexp_matches
> ----------------
> {abc,0,""}
> (1 row)
>
> regression=# select regexp_matches('abc01234xyz', '\0*(.*?)(\d+)(.*)');
> regexp_matches
> -----------------
> {abc,01234,xyz}
> (1 row)
>
>
​+1

David J.​