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You are right, and I should have checked before opening this. The lookahead is doing real work — it bounds how far each match attempt can scan, so removing it makes the pattern quadratic. Measured on Node 24, input
Four times the work per doubling, so the current guard is the thing keeping this linear. The divergence from the HTML tokenizer on a nested Closing. |
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Prism's
commentrule for markup uses/<!--(?:(?!<!--)[\s\S])*?-->/, which refuses to match once it encounters a second<!--inside the comment body. Per the HTML tokenizer (WHATWG §13.2.5 comment states), a comment ends at the first-->; an embedded<!--has no special meaning and is just content.So
<!-- foo <!-- bar -->— commented-out markup that itself contains a comment marker — fails to tokenize as a comment and falls through to thetagrule, which mis-highlights the whole thing as a bogus HTML tag. Dropping the(?!<!--)guard matches the comment as a single token, in line with the spec. Added a test case.