added support for simple references

fixes #33

Markdown.php

@@ -703,12 +703,12 @@ private function parseLinkOrImage($markdown)
 					empty($refMatches[5]) ? null: $refMatches[5], // title
 					$offset + strlen($refMatches[0]), // offset
 				];
-			} elseif (preg_match('/^[ \n]?\[(.*?)\]/', $markdown, $refMatches)) {
+			} elseif (preg_match('/^[ \n]?(\[(.*?)\])?/', $markdown, $refMatches)) {
 				// reference style link
-				if (empty($refMatches[1])) {
+				if (empty($refMatches[2])) {
 					$key = strtolower($text);
 				} else {
-					$key = strtolower($refMatches[1]);
+					$key = strtolower($refMatches[2]);
 				}
 				if (isset($this->references[$key])) {
 					return [

tests/github-data/issue-33.html

@@ -0,0 +1,5 @@
+<pre><code>hey, check [this].
+
+[this]: https://github.com/cebe/markdown
+</code></pre>
+<p>is a vaild reference.</p>

tests/github-data/issue-33.md

@@ -0,0 +1,6 @@
+```
+hey, check [this].
+
+[this]: https://github.com/cebe/markdown
+```
+is a vaild reference.

tests/markdown-data/references.html

@@ -1,2 +1,3 @@
-<p>In the following example we will implement support for <a href="https://help.github.com/articles/github-flavored-markdown#fenced-code-blocks" title="Fenced code block feature of github flavored markdown">fenced code blocks</a> which are part
+<p>In the following example we will im[ple]ment support for <a href="https://help.github.com/articles/github-flavored-markdown#fenced-code-blocks" title="Fenced code block feature of github flavored markdown">fenced code blocks</a> which are part
 of the <a href="https://github.com">github flavored markdown</a>.</p>
+<p>hey, check <a href="https://github.com/cebe/markdown">this</a>.</p>

tests/markdown-data/references.md

@@ -1,6 +1,11 @@
-In the following example we will implement support for [fenced code blocks][] which are part
+In the following example we will im[ple]ment support for [fenced code blocks][] which are part
 of the [github flavored markdown][gfm].
 
 [fenced code blocks]: https://help.github.com/articles/github-flavored-markdown#fenced-code-blocks
                       "Fenced code block feature of github flavored markdown"
 [gfm]: https://github.com
+[unused]: https://github.com/unused
+
+hey, check [this].
+
+[this]: https://github.com/cebe/markdown