<regex>: Fix word bounds \b and \B when applied to the empty string

[muellerj2]

Fixes #5371.

The ECMAScript standard effectively states that \b matches a position i in the string iff IsWordChar(i-1) xor IsWordChar(i) is true. It also defines that for positions i = -1 and i = length(input), IsWordChar(i) always returns false. Thus, \b does not match anything in the empty string.

Since \b does not match the empty string, the flags match_not_bow and match_not_eow don't matter in this case.

[StephanTLavavej]

Thanks for the fix, test coverage, and clear writeup with citations! I pushed a trivial change to use raw string literals.

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

Resolved adjacent-add conflict in VSO_0000000_regex_use.

[StephanTLavavej]

🐝 🪲 🐞