`<regex>`: Consider special-casing some simple loop structures

[muellerj2]

Currently, loops like a* or \d* go through the whole NFA state transition processing for each repetition, which slows down the handling of such loops a lot.

I think it's probably worth it to recognize and special-case such simple and comonly used loop structures in the matcher. (But we should not add such special-case handling to non-simple loops and instead resolve #5957 to effectively extend the simple loop handling to branchless loops.)

[StephanTLavavej]

Agreed. I'd expect zero-or-more / one-or-more repeats of space / whitespace being the most common, followed by similar loops to absorb digits (as you mentioned), word characters (I use \w+ all the time), or handwritten classes like [a-zA-Z]+.