Added a workaround for a PHP Bug

- https://bugs.php.net/bug.php?id=45735
- http://www.regular-expressions.info/catastrophic.html
- erusev/parsedown#443

CHANGELOG.md

@@ -4,7 +4,12 @@ CHANGELOG
 Version 1.1.2 work in progress
 ------------------------------
 
-...
+- Added a workaround for a [PHP bug](https://bugs.php.net/bug.php?id=45735) which exists in versions `<` 7.0, where `preg_match()` causes a segfault
+  on [catastropic backtracking][] in emph/strong parsing.
+
+
+[catastropic backtracking]: http://www.regular-expressions.info/catastrophic.html
+
 
 Version 1.1.1 work in progress
 ------------------------------

inline/EmphStrongTrait.php

@@ -26,6 +26,14 @@ protected function parseEmphStrong($text)
 		}
 
 		if ($marker == $text[1]) { // strong
+			// work around a PHP bug that crashes with a segfault on too much regex backtrack
+			// check whether the end marker exists in the text
+			// https://github.com/erusev/parsedown/issues/443
+			// https://bugs.php.net/bug.php?id=45735
+			if (strpos($text, $marker . $marker, 2) === false) {
+				return [['text', $text[0] . $text[1]], 2];
+			}
+
 			if ($marker == '*' && preg_match('/^[*]{2}((?:[^*]|[*][^*]*[*])+?)[*]{2}(?![*]{2})/s', $text, $matches) ||
 				$marker == '_' && preg_match('/^__((?:[^_]|_[^_]*_)+?)__(?!__)/us', $text, $matches)) {
 
@@ -38,6 +46,14 @@ protected function parseEmphStrong($text)
 				];
 			}
 		} else { // emph
+			// work around a PHP bug that crashes with a segfault on too much regex backtrack
+			// check whether the end marker exists in the text
+			// https://github.com/erusev/parsedown/issues/443
+			// https://bugs.php.net/bug.php?id=45735
+			if (strpos($text, $marker, 1) === false) {
+				return [['text', $text[0]], 1];
+			}
+
 			if ($marker == '*' && preg_match('/^[*]((?:[^*]|[*][*][^*]+?[*][*])+?)[*](?![*][^*])/s', $text, $matches) ||
 				$marker == '_' && preg_match('/^_((?:[^_]|__[^_]*__)+?)_(?!_[^_])\b/us', $text, $matches)) {
 				return [