Re: Patch: regexp_matches variant returning an array of matching positions

I'll elaborate on the use case. I have OCR scanned text for a large amounts
of images, corresponding to one row per image. I want to match against
words in another table. I need two results sets, one with all matched words
and one with only the first matched word within the first 50 chars of the
OCR scanned text. Having the matched position in the first result set makes
it easy to produce the second.

I cannot find the position using the substring because I use word
boundaries in my regexp.

Returning a SETOF named composite makes sense, so I could try to make such
a function instead if there is interest. Perhaps a good name for such a
function would be simply regexp_match och regexp_search (as in python).

/Björn

2014-01-29 David Johnston <polobo(at)yahoo(dot)com>

> Alvaro Herrera-9 wrote
> > Björn Harrtell wrote:
> >> I've written a variant of regexp_matches called regexp_matches_positions
> >> which instead of returning matching substrings will return matching
> >> positions. I found use of this when processing OCR scanned text and
> >> wanted
> >> to prioritize matches based on their position.
> >
> > Interesting. I didn't read the patch but I wonder if it would be of
> > more general applicability to return more info in a fell swoop a
> > function returning a set (position, length, text of match), rather than
> > an array. So instead of first calling one function to get the match and
> > then their positions, do it all in one pass.
> >
> > (See pg_event_trigger_dropped_objects for a simple example of a function
> > that returns in that fashion. There are several others but AFAIR that's
> > the simplest one.)
>
> Confused as to your thinking. Like regexp_matches this returns "SETOF
> type[]". In this case integer but text for the matches. I could see
> adding
> a generic function that returns a SETOF named composite (match varchar[],
> position int[], length int[]) and the corresponding type. I'm not
> imagining
> a situation where you'd want the position but not the text and so having to
> evaluate the regexp twice seems wasteful. The length is probably a waste
> though since it can readily be gotten from the text and is less often
> needed. But if it's pre-calculated anyway...
>
> My question is what position is returned in a multiple-match situation? The
> supplied test only covers the simple, non-global, situation. It needs to
> exercise empty sub-matches and global searches. One theory is that the
> first array slot should cover the global position of match zero (i.e., the
> entire pattern) within the larger document while sub-matches would be
> relative offsets within that single match. This conflicts, though, with
> the
> fact that _matches only returns array elements for () items and never for
> the full match - the goal in this function being parallel un-nesting. But
> as
> nesting is allowed it is still possible to have occur.
>
> How does this resolve in the patch?
>
> SELECT regexp_matches('abcabc','((a)(b)(c))','g');
>
> David J.
>
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