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]".

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Issue Details

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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