Regex compiler/ source generator can use {Last}IndexOf for strings in greedy char loop

[stephentoub]

In greedy char loops, if the loop is followed by a known literal, when backtracking we LastIndexOf to find the next possible place to try to continue the match, rather than just backing up one char at a time. Today we only do this based on a subsequent char literal, but we can extend it to look for a subsequent multi as well. We can also build a set of all possible subsequent chars and do a LastIndexOfAny, e.g. for ".*[abc]".

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[ghost]

Tagging subscribers to this area: @dotnet/area-system-text-regularexpressions
See info in area-owners.md if you want to be subscribed.

Issue Details

---

In greedy char loops, if the loop is followed by a known literal, when backtracking we LastIndexOf to find the next possible place to try to continue the match, rather than just backing up one char at a time. Today we only do this based on a subsequent char literal, but we can extend it to look for a subsequent multi as well. (We could even build a set of all possible subsequent chars and do a LastIndexOfAny, though the benefits of that appear to be less common.)

Author:	stephentoub
Assignees:	-
Labels:	area-System.Text.RegularExpressions, tenet-performance, untriaged
Milestone:	7.0.0