Adding regexp_match() function

Hello,

I'd like to contribute a "regexp_match()" function as discussed in bug
#5469 [1] The aim is to overcome the limitation outlined in the thread
above <http://archives.postgresql.org/pgsql-bugs/2010-05/msg00227.php>.

PostgreSQL currently offers the function regexp_matches(), a SRF
(which, unless invoked with the 'g' flag, returns at most one result).
An user interested in the "nonglob" behaviour, i.e. expecting at most
1 match group, has to adopt special care to avoid records to be
dropped from the target list in case no match is found. Being this a
rather common use case, I'd like to provide a function with a less
astonishing behaviour, i.e. returning a text[] instead of a set of
text[] and returning NULL in case no match is found (the 'g' flag
wouldn't be supported).

The proposed name is regexp_match(), to underline the semantics very
similar to regexp_matches() but returning a single value as result.
While the symmetry between the names is a pro, an excessive similarity
may result confusing, so I wouldn't be surprised if a better name is
found. The implementation of the function is very simple, given the
infrastructure already available for the other regexp-related
functions. I've actually already implemented it (mostly to check how
easy or hard it would have been: I had never written a C procedure for
PG before): a preliminary patch is attached.

If the idea is accepted I will submit a complete patch including
documentation and tests.

Best regards,

-- Daniele