Perl-compatible unsetting of captures during repeats

[NWilson]

In maint/README:

Perl and PCRE2 sometimes differ in the settings of capturing subpatterns
inside repeats. One example of the difference is the matching of
/(main(O)?)+/ against mainOmain, where PCRE2 leaves $2 set. In Perl, it's
unset. Changing this in PCRE2 will be very hard because I think it needs much
more state to be remembered.

In pcre2compat:

13. There are some differences that are concerned with the settings of captured
    strings when part of a pattern is repeated. For example, matching "aba" against
    the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE2 it is set to
    "b".

This seems to be the most major thing in pcre2compat. It's also the only definite bug listed in maint/README (the rest seem to be fairly minor feature requests).

Unlike the technicalities of (*THEN) inside recursive patterns, or other trivia, that has major impact on "simple" regexes that just use standard syntax, like "abacb" =~ /^(a(b)?)+c\2$/ (as mentioned above).

[PhilipHazel]

Interestingly, nobody before has ever commented on these differences. I conclude that they are not in practice important.

[NWilson]

I at least discovered this difference before noticing it in pcre2compat (and I didn't find maint/README until much later).

I was doing a very thorough investigation of behavioural differences between regex engines, in order to inform the behaviour choices in the Excel API.

I agree users are unlikely to notice or care.