fix: Recognize setext headings whose first line starts with ##4015
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A line like `#1 Goals` or `#hashtag` followed by a setext underline was rendered as a paragraph plus `<hr>` instead of a heading. A line only starts an ATX heading when the `#`s are followed by whitespace or EOL, so these are paragraph text and the underline makes a setext heading. The lheading ATX-interrupt check was bare ` {0,3}#{1,6}`, missing the `(?:\s|$)` that the paragraph and ATX heading rules already require.
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styfle
approved these changes
Jul 14, 2026
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A line like
#1 Goalsor#hashtagfollowed by a setext underline is rendered as a paragraph plus an<hr>instead of a heading:Both commonmark.js and markdown-it produce
<h2>#1 Goals</h2>here, and that matches the CommonMark spec: a line only starts an ATX heading when the#s are followed by a space, a tab, or the end of the line, so#1is ordinary paragraph text — which means the---------beneath it makes a setext heading.The
lheadingrule already knows other blocks can interrupt a setext heading, but its ATX check is{0,3}#{1,6}, which matches any line that merely starts with#and misses the "followed by whitespace" requirement. marked's own paragraph rules already get this right ({0,3}#{1,6}(?:\s|$)), as does the ATXheadingrule itself ((?=\s|$)); this just bringslheading/lheadingGfmin line with them.Valid ATX headings are unaffected —
# Real\n===still renders<h1>Real</h1>followed by<p>===</p>— and the full spec suite passes with the added fixture.