Add test for domains with underscores and fix roundtrip behavior.

Author: kevinbackhouse

extensions/autolink.c

@@ -116,7 +116,20 @@ static size_t autolink_delim(uint8_t *data, size_t link_end) {
 static size_t check_domain(uint8_t *data, size_t size, int allow_short) {
   size_t i, np = 0, uscore1 = 0, uscore2 = 0;
 
+  /* The purpose of this code is to reject urls that contain an underscore
+   * in one of the last two segments. Examples:
+   *
+   *   www.xxx.yyy.zzz     autolinked
+   *   www.xxx.yyy._zzz    not autolinked
+   *   www.xxx._yyy.zzz    not autolinked
+   *   www._xxx.yyy.zzz    autolinked
+   *
+   * The reason is that domain names are allowed to include underscores,
+   * but host names are not. See: https://stackoverflow.com/a/2183140
+   */
   for (i = 1; i < size - 1; i++) {
+    if (data[i] == '\\' && i < size - 2)
+      i++;
     if (data[i] == '_')
       uscore2++;
     else if (data[i] == '.') {

test/extensions.txt

@@ -581,6 +581,12 @@ www.github.com www.github.com/á
 
 www.google.com/a_b
 
+Underscores not allowed in host name www.xxx.yyy._zzz
+
+Underscores not allowed in host name www.xxx._yyy.zzz
+
+Underscores allowed in domain name www._xxx.yyy.zzz
+
 **Autolink and http://inlines**
 
 ![http://inline.com/image](http://inline.com/image)
@@ -618,6 +624,9 @@ http://🍄.ga/ http://x🍄.ga/
 <p>Email me at:<a href="mailto:[email protected]">[email protected]</a></p>
 <p><a href="http://www.github.com">www.github.com</a> <a href="http://www.github.com/%C3%A1">www.github.com/á</a></p>
 <p><a href="http://www.google.com/a_b">www.google.com/a_b</a></p>
+<p>Underscores not allowed in host name www.xxx.yyy._zzz</p>
+<p>Underscores not allowed in host name www.xxx._yyy.zzz</p>
+<p>Underscores allowed in domain name <a href="http://www._xxx.yyy.zzz">www._xxx.yyy.zzz</a></p>
 <p><strong>Autolink and <a href="http://inlines">http://inlines</a></strong></p>
 <p><img src="http://inline.com/image" alt="http://inline.com/image" /></p>
 <p><a href="mailto:[email protected]">[email protected]</a></p>