Fix length check for Regex BOL FindFirstChar optimization

[stephentoub]

For a beginning-of-line anchor, in FindFirstChar we use IndexOf to quickly skip ahead to the next \n. But we neglected to check to see whether that brought us past an explicitly specified end position. This just adds the missing check.

Fixes#55557
cc: @strobelleder, @dotnet/area-system-text-regularexpressions

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Issue Details

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For a beginning-of-line anchor, in FindFirstChar we use IndexOf to quickly skip ahead to the next \n. But we neglected to check to see whether that brought us past an explicitly specified end position. This just adds the missing check.

Fixes #55557
cc: @strobelleder, @dotnet/area-system-text-regularexpressions

Author:	stephentoub
Assignees:	-
Labels:	area-System.Text.RegularExpressions
Milestone:	-

[eerhardt]

LGTM

Do you think this warrants a backport to 5.0?

[stephentoub]

Do you think this warrants a backport to 5.0?

It's a simple fix, so I'm not against backporting it. But it also requires a pretty special combination of things (a ^ anchor with RegexOptions.Multiline and specifying an end position to the match that's before a \n), so I don't expect it'll impact too many folks.

[pgovind]

LGTM