Fast forward look-around threads upon prefiltering

[shilangyu]

No description provided.

[Copilot]

Pull Request Overview

This PR refactors the look-behind processing logic within the PikeVM by consolidating duplicated looping code into a new helper function, fast_forward_lookbehinds, and updating its usage in matching and prefilter branches.

• Replaces manual loops with a dedicated fast_forward_lookbehinds function.
• Updates both the initial matching and the pre-filter branch to leverage the new helper.
• Improves maintainability by reducing code duplication in look-behind thread processing.

Comments suppressed due to low confidence (1)

regex-automata/src/nfa/thompson/pikevm.rs:1480

• [nitpick] Consider adding or updating tests to cover edge cases for fast_forward_lookbehinds, such as when input.start is 0 or when the forward span covers the entire valid range.

for lb_at in forward_span.start..forward_span.end {

[Multimodcrafter]

Looks like a clean fix to me. Of course, the prefiltering might not be so relevant if the look-behind threads have to be stepped through but on the other hand, look-behind expressions might be much simpler than the main regex and in that case, it could have a performance benefit.