diff --git a/link.module b/link.module
index b5a42ab..dd231d1 100644
--- a/link.module
+++ b/link.module
@@ -1156,7 +1156,7 @@ function link_validate_url($text) {
   // Starting a parenthesis group with (?: means that it is grouped, but is not captured
   $protocol = '((?:'. implode("|", $allowed_protocols) .'):\/\/)';
   $authentication = "(?:(?:(?:[\w\.\-\+!$&'\(\)*\+,;=" . $LINK_ICHARS . "]|%[0-9a-f]{2})+(?::(?:[\w". $LINK_ICHARS ."\.\-\+%!$&'\(\)*\+,;=]|%[0-9a-f]{2})*)?)?@)";
-  $domain = '(?:(?:[a-z0-9' . $LINK_ICHARS_DOMAIN . ']([a-z0-9'. $LINK_ICHARS_DOMAIN . '\-_\[\]])*)(\.(([a-z0-9' . $LINK_ICHARS_DOMAIN . '\-_\[\]])+\.)*('. LINK_DOMAINS .'|[a-z]{2}))?)';
+  $domain = '(?:(?:[a-z0-9_' . $LINK_ICHARS_DOMAIN . ']([a-z0-9'. $LINK_ICHARS_DOMAIN . '\-_\[\]])*)(\.(([a-z0-9' . $LINK_ICHARS_DOMAIN . '\-_\[\]])+\.)*('. LINK_DOMAINS .'|[a-z]{2}))?)';
   $ipv4 = '(?:[0-9]{1,3}(\.[0-9]{1,3}){3})';
   $ipv6 = '(?:[0-9a-fA-F]{1,4}(\:[0-9a-fA-F]{1,4}){7})';
   $port = '(?::([0-9]{1,5}))';