Re: Re: [PATCH] regexp_positions ( string text, pattern text, flags text ) → setof int4range[]

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: "Joel Jacobson" <joel(at)compiler(dot)org>
Cc: "daniel(at)yesql(dot)se" <daniel(at)yesql(dot)se>, "Mark Dilger" <mark(dot)dilger(at)enterprisedb(dot)com>, "Postgres hackers" <pgsql-hackers(at)lists(dot)postgresql(dot)org>, "Andreas Karlsson" <andreas(at)proxel(dot)se>, "David Fetter" <david(at)fetter(dot)org>, "Gilles Darold" <gilles(at)darold(dot)net>, "Pavel Stehule" <pavel(dot)stehule(at)gmail(dot)com>
Subject: Re: Re: [PATCH] regexp_positions ( string text, pattern text, flags text ) → setof int4range[]
Date: 2021-09-21 17:55:24
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"Joel Jacobson" <joel(at)compiler(dot)org> writes:
> [ 0005-regexp-positions.patch ]

I took a quick look at this patch. I am on board now with the general
idea of returning match positions, but I think there are still
definitional issues to be discussed.

1. The main idea we might perhaps want to adopt from the Oracle-ish
regexp functions is a search-start-position argument. I'm not sure
that this is exciting --- if we intend this function to be a close
analog of regexp_matches(), it'd be better to leave that off. But
it deserves explicit consideration.

2. It looks like you modeled this on regexp_matches() to the extent
of returning a set and using the 'g' flag to control whether the
set can actually contain more than one row. That's pretty ancient
precedent ... but we have revisited that design more than once
because regexp_matches() is just too much of a pain to use. I think
if we're going to do this, we should learn from history, and provide an
analog to regexp_match() as well as regexp_matches() right off the bat.

3. The API convention you chose (separate start and length arrays)
is perhaps still confusing. When I first looked at the test case

+SELECT regexp_positions('foobarbequebaz', $re$(bar)(beque)$re$);
+ regexp_positions
+ ("{4,7}","{3,5}")
+(1 row)

I thought it was all wrong because it seemed to be identifying
the substrings 'barbequ' and 'obarb'. If there'd been a different
number of matches than capture groups, maybe I'd not have been
confused, but still... I wonder if we'd be better advised to make
N capture groups produce N two-element arrays, or else mash it all
into one array of N*2 elements. But this probably depends on which
way is the easiest to work with in SQL.

4. Not sure about the handling of sub-matches.
There are various plausible definitions we could use:

* We return the position/length of the overall match, never mind
about any parenthesized subexpressions. This is simple but I think
it loses significant functionality. As an example, you might have
a pattern like 'ab*(c*)d+' where what you actually want to know
is where the 'c's are, but they have to be in the context described
by the rest of the regexp. Without subexpression-match capability
that's painful to do.

* If there's a capturing subexpression, return the position/length
of the first such subexpression, else return the overall match.
This matches the behavior of substring().

* If there are capturing subexpression(s), return the
positions/lengths of those, otherwise return the overall match.
This agrees with the behavior of regexp_match(es), so I'd tend
to lean to this option, but perhaps it's the hardest to use.

* Return the position/length of the overall match *and* those of
each capturing subexpression. This is the most flexible choice,
but I give it low marks since it matches no existing behaviors.

As for comments on the patch itself:

* The documentation includes an extraneous entry for regexp_replace,
and it fails to add the promised paragraph to functions-posix-regexp.

* This bit is evidently copied from regexp_matches:

+ /* be sure to copy the input string into the multi-call ctx */
+ matchctx = setup_regexp_matches(PG_GETARG_TEXT_P_COPY(0), pattern,

regexp_matches needs to save the input string so that
build_regexp_match_result can copy parts of that. But
regexp_positions has no such need AFAICS, so I think we
don't need a long-lived copy of the string.

* I wouldn't use these names in build_regexp_positions_result:

+ ArrayType *so_ary;
+ ArrayType *eo_ary;

"so_ary" isn't awful, but it invites confusion with regex's "so"
field, which hasn't got the same semantics (off by one).
"eo_ary" is pretty bad because it isn't an "end offset" at all, but
a length. I'd go for "start_ary" and "length_ary" or some such.

* Test cases seem a bit thin, notably there's no coverage of the
null-subexpression code path.

regards, tom lane

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