Fix catastrophic backtracking when parsing link labels/titles

Author: colinodell

CHANGELOG.md

@@ -105,6 +105,7 @@ Updates should follow the [Keep a CHANGELOG](https://keepachangelog.com/) princi
 - Fixed quadratic complexity finding the bottom opener for emphasis and strikethrough delimiters
 - Fixed issue where having 500,000+ delimiters could trigger a [known segmentation fault issue in PHP's garbage collection](https://bugs.php.net/bug.php?id=68606)
 - Fixed quadratic complexity deactivating link openers
+- Fixed catastrophic backtracking when parsing link labels/titles
 
 ## [2.4.1] - 2023-08-30
 

src/Util/LinkParserHelper.php

@@ -69,7 +69,7 @@ public static function parseLinkLabel(Cursor $cursor): int
 
     public static function parsePartialLinkLabel(Cursor $cursor): ?string
     {
-        return $cursor->match('/^(?:[^\\\\\[\]]+|\\\\.?)*/');
+        return $cursor->match('/^(?:[^\\\\\[\]]++|\\\\.?)*+/');
     }
 
     /**

src/Util/RegexHelper.php

@@ -61,9 +61,9 @@ final class RegexHelper
         self::PARTIAL_PROCESSINGINSTRUCTION . '|' . self::PARTIAL_DECLARATION . '|' . self::PARTIAL_CDATA . ')';
     public const PARTIAL_HTMLBLOCKOPEN         = '<(?:' . self::PARTIAL_BLOCKTAGNAME . '(?:[\s\/>]|$)' . '|' .
         '\/' . self::PARTIAL_BLOCKTAGNAME . '(?:[\s>]|$)' . '|' . '[?!])';
-    public const PARTIAL_LINK_TITLE            = '^(?:"(' . self::PARTIAL_ESCAPED_CHAR . '|[^"\x00])*"' .
-        '|' . '\'(' . self::PARTIAL_ESCAPED_CHAR . '|[^\'\x00])*\'' .
-        '|' . '\((' . self::PARTIAL_ESCAPED_CHAR . '|[^()\x00])*\))';
+    public const PARTIAL_LINK_TITLE            = '^(?:"(' . self::PARTIAL_ESCAPED_CHAR . '|[^"\x00])*+"' .
+        '|' . '\'(' . self::PARTIAL_ESCAPED_CHAR . '|[^\'\x00])*+\'' .
+        '|' . '\((' . self::PARTIAL_ESCAPED_CHAR . '|[^()\x00])*+\))';
 
     public const REGEX_PUNCTUATION        = '/^[!"#$%&\'()*+,\-.\\/:;<=>?@\\[\\]\\\\^_`{|}~\p{P}\p{S}]/u';
     public const REGEX_UNSAFE_PROTOCOL    = '/^javascript:|vbscript:|file:|data:/i';