## Summary
Replace regex patterns with lookahead/lookbehind assertions to prevent
catastrophic backtracking on Ruby 3.1 and earlier.
This issue only affects Ruby 3.1 and earlier. Since it does not occur on Ruby 3.2+,
it is not an issue in the current stable release (0.4.0), which does not include #206.
## Regression Testing
Ensure the regex patterns complete within 1 second for a 50,000-character input
that would otherwise trigger superlinear backtracking.
248b4d2dad