Share the CommonMark grammar, close the guard's hole, refuse the lists no marker spells

src/commonmark-grammar.ts

@@ -0,0 +1,31 @@
+export type LinePosition = 'first' | 'later'
+
+const entityReferenceSource = '&(?:[A-Za-z][A-Za-z0-9]{1,31}|#\\d{1,7}|#[Xx][A-Fa-f0-9]{1,6});'
+
+const anchoredEntityReference = new RegExp(`^(?:${entityReferenceSource})`)
+const entityReference = new RegExp(entityReferenceSource)
+const firstCharacterOpeners = [/^#{1,6}(?:[ \t]|$)/, /^>/, /^[*+-](?:[ \t]|$)/, /^`{3,}/, /^~{3,}/, /^:{2,}/, /^\|/]
+const orderedListOpener = /^(\d{1,9})[.)](?:[ \t]|$)/
+const setextUnderline = /^=+$/
+const thematicBreak = /^(?:(?:\*[ \t]*){3,}|(?:-[ \t]*){3,}|(?:_[ \t]*){3,})$/
+
+export function claimsLine(line: string, position: LinePosition): boolean {
+  return escapesLineClaim(line, 0, position) || orderedListOpener.test(line)
+}
+
+export function escapesLineClaim(line: string, offset: number, position: LinePosition): boolean {
+  if (offset === 0) {
+    if (firstCharacterOpeners.some((opener) => opener.test(line)) || thematicBreak.test(line)) return true
+    return position === 'later' && setextUnderline.test(line)
+  }
+  const digits = orderedListOpener.exec(line)?.[1]
+  return digits !== undefined && offset === digits.length
+}
+
+export function holdsEntityReference(text: string): boolean {
+  return entityReference.test(text)
+}
+
+export function startsEntityReference(text: string): boolean {
+  return anchoredEntityReference.test(text)
+}