fix: prevent quadratic complexity in emStrongLDelim regex

Make suffix groups optional to eliminate O(n) backtracking per exec().
Skip entire delimiter run as text when no valid suffix is found to
avoid the O(n^2) amplification loop.

Author: sammiee5311

src/Tokenizer.ts

@@ -726,14 +726,22 @@ export class _Tokenizer<ParserOutput = string, RendererOutput = string> {
     }
   }
 
-  emStrong(src: string, maskedSrc: string, prevChar = ''): Tokens.Em | Tokens.Strong | undefined {
+  emStrong(src: string, maskedSrc: string, prevChar = ''): Tokens.Em | Tokens.Strong | Tokens.Text | undefined {
     let match = this.rules.inline.emStrongLDelim.exec(src);
     if (!match) return;
+    if (!match[1] && !match[2] && !match[3] && !match[4]) {
+      // Delimiter run has no valid suffix — skip entire run as text
+      return {
+        type: 'text',
+        raw: match[0],
+        text: match[0],
+      };
+    }
 
     // _ can't be between two alphanumerics. \p{L}\p{N} includes non-english alphabet/numbers as well
-    if (match[3] && prevChar.match(this.rules.other.unicodeAlphaNumeric)) return;
+    if (match[4] && prevChar.match(this.rules.other.unicodeAlphaNumeric)) return;
 
-    const nextChar = match[1] || match[2] || '';
+    const nextChar = match[1] || match[3] || '';
 
     if (!nextChar || !prevChar || this.rules.inline.punctuation.exec(prevChar)) {
       // unicode Regex counts emoji as 1 char; spread into array for proper count (used multiple times below)

src/rules.ts

@@ -290,7 +290,7 @@ const blockSkip = edit(/link|precode-code|html/, 'g')
   .replace('html', /<(?! )[^<>]*?>/)
   .getRegex();
 
-const emStrongLDelimCore = /^(?:\*+(?:((?!\*)punct)|[^\s*]))|^_+(?:((?!_)punct)|([^\s_]))/;
+const emStrongLDelimCore = /^(?:\*+(?:((?!\*)punct)|([^\s*]))?)|^_+(?:((?!_)punct)|([^\s_]))?/;
 
 const emStrongLDelim = edit(emStrongLDelimCore, 'u')
   .replace(/punct/g, _punctuation)

test/specs/redos/quadratic_emstrong_delim.cjs

@@ -0,0 +1,4 @@
+module.exports = {
+  markdown: '_'.repeat(10000) + ' a',
+  html: `<p>${'_'.repeat(10000)} a</p>\n`,
+};