Fix RegEx search_all for zero length matches/lookahead

[TheSofox]

Fixes #85605, fixes #73920

'search_all' in RegEx function wasn't handling zero length matches well. When one was found, it would stop and not search for any more matches. My fix simply increments the position in the string after a zero length match and proceeds as normal otherwise.

This fixes the lookahead problem in the linked issue, as lookaheads are one way of having zero lenght matches that still return a value.

Of note, I've also attached a few extra RegEx tests that check for lookahead functions, include the specific example in the linked issue.

[TheSofox]

Damn, just experimented and noticed that this doesn't handle zero length lookbehinds very well. To be fair, the previous code also breaks at zero length lookbehinds, my code just breaks it in a different (and better?) way.

I'll see if I can amend this PR to take that into account, along with some more unit tests.

[TheSofox]

And pushed. Managed to find a solution that worked for zero length results, whether lookahead or lookbehind. Added lookbehind tests too.

While the original bug report didn't explicitly state lookbehind as an issue, it still falls under the same root issue that this PR is trying to fix (ie. Zero length results not being handled gracefully by 'search_all')

[TheSofox]

Came across another bug that this PR fixes already (this one: #73920 ). Gonna add another test case to this PR the covers the circumstance that the bug lists.

[adamscott]

Tested all the test cases with https://regex101.com/. It matches.

And I approve the change of logic of RegEx::search_all(), it makes more sense now and it shows how the previous logic was flawed. It could only return 1 result if it got any zero-length match.

[akien-mga]

Should the documentation for RegEx be amended to clarify the change in behavior?

[adamscott]

Should the documentation for RegEx be amended to clarify the change in behavior?

No? From what I read, the documentation is OK, it's just that it was bugged for some usecases, which this PR fixes.

I'll take a look if it's possible to cherry-pick this PR for 4.1.

[akien-mga]

Thanks!

[thebluetropics]

hey thanks for the fix! 🎉

[YuriSizov]

Cherry-picked for 4.2.2.