diff --git a/lib/setup.php b/lib/setup.php
index bba85068bb3..aabf33d468c 100644
--- a/lib/setup.php
+++ b/lib/setup.php
@@ -387,6 +387,10 @@ global $HTTPSPAGEREQUIRED;
 
     @ini_set('arg_separator.output', '&');
 
+/// Work around for a PHP bug   see MDL-11237
+  
+    @ini_set('pcre.backtrack_limit', 20971520);  // 20 MB 
+
 /// Location of standard files
 
     $CFG->wordlist    = $CFG->libdir .'/wordlist.txt';