BUG #15595: matchctx->npatterns is always 1 no matter pattern have 0or 1 subexpression

The following bug has been logged on the website:

Bug reference:      15595
Logged by:          Damion
Email address:      zhq651@126.com
PostgreSQL version: 11.1
Operating system:   linux
Description:

Hi,
when I use setup_regexp_matches, I found matchctx->npatterns is always 1 no
matter pattern  have not or one subexpr subexpression.
so I can't tell whether there is a subexpression. see below:

 if orig_str is '221133' & pattern  is  '11'
(gdb) p nth_subexpr
$23 = 1
(gdb) p matchctx->npatterns
$24 = 1

 if orig_str is '221133' & pattern  is  '(11)'
(gdb) p nth_subexpr
$25 = 1
(gdb) p matchctx->npatterns
$26 = 1

if pattern  is  '((1)1)' or '((11))' (2 subexpression  or more), I can kwown
how many subexpression by matchctx->npatterns .
postgres=# SELECT regexp_substr('221133', '((1)1)', 1, 1, 'i', 1);
(gdb) p matchctx->npatterns
$27 = 2
(gdb) p nth_subexpr
$28 = 1

Thanks.