According to the debugger at http://regex101.com/:

  /.*(\d)/

- Searches right-to-left, backtracking until it finds the match.

  /(\d)\D*$/

- Searches left-to-right, going forward a step, backtrack, forward, backtrack, until it finds a match.

If you're looking for a match toward the end of a string, the .* version will be faster.