Fix Markdown handling of # characters

- remove incorrect logic to identify headers
- ensure header detection abides by the CommonMark spec rules [1]
- remove isBugLink method, which is no longer needed since bug links
  (i.e. `#number`) are not recognized as a header by the new regex

Fixes #22333

[1] http://spec.commonmark.org/0.27/#atx-headings

Signed-off-by: Damien Regad <dregad@mantisbt.org>

Changes to Joel's original code: squashed 2 commits, reworded commit
message as well as the comments within the code.

plugins/MantisCoreFormatting/core/MantisMarkdown.php

@@ -97,52 +97,21 @@ public static function convert_line( $p_text ) {
 	}
 
 	/**
-	 * Customize the logic on Header elements
+	 * Customize the blockHeader markdown
 	 *
 	 * @param string $line The Markdown syntax to parse
 	 * @access protected
-	 * @return string|null HTML representation generated from markdown or
-	 *                     null if markdown starts with # symbol
+	 * @return string|null HTML representation generated from markdown or null
+	 *                if text is not a valid header per CommonMark spec
 	 */
 	protected function blockHeader( $line ) {
-		$block = parent::blockHeader( $line );
-
-		# Bug links should not be treated as headers
-		if( $this->isBugLink( $line['text'] ) ) {
-			return null;
-		}
-
-		# Header rules
-		# hash[space][numbers] - treated as header
-		# hash[number][*] - treated as header since it is not a pure number
-		# hash[letter][*] - treated as header
-		# hash[space][letter][*] - treated as header
-		return $block;
-	}
-
-	/**
-	 * Customize the logic on setting the Header elements.
-	 *
-	 * @param string $line The Markdown syntax to parse
-	 * @param array $block A block-level element
-	 * @access protected
-	 * @return string|null HTML representation generated from markdown or
-	 *                     null if markdown starts with # symbol
-	 */
-	protected function blockSetextHeader( $line, array $block = null ) {
-		$block = parent::blockSetextHeader( $line, $block );
-
-		# Bug links should not be treated as headers
-		if( $this->isBugLink( $line['text'] ) ) {
-			return null;
+		# Header detection logic
+		# - the opening # may be indented 0-3 spaces
+		# - a sequence of 1–6 '#' characters
+		# - The #'s must be followed by a space or a newline
+		if ( preg_match( '/^ {0,3}#{1,6}(?: |$)/', $line['text'] ) ) {
+			return parent::blockHeader($line);
 		}
-
-		# Header rules
-		# hash[space][numbers] - treated as header
-		# hash[number][*] - treated as header since it is not a pure number
-		# hash[letter][*] - treated as header
-		# hash[space][letter][*] - treated as header
-		return $block;
 	}
 
 	/**
@@ -325,15 +294,4 @@ private function processAmpersand( &$p_text ) {
 		$p_text = str_replace( '&', '&', $p_text );
 	}
 
-	/**
-	 * Check if the given string is a bug link reference.
-	 *
-	 * @param string $p_text
-	 * @return bool
-	 */
-	private function isBugLink( $p_text ) {
-		return '#' == config_get_global( 'bug_link_tag' )
-			&& preg_match_all( '/^#\d+$/', $p_text, $matches );
-	}
-
 }