MDL-43000 shortanswer qtype: handle patterns with many *s

Teachers were typing patterns like
********************************<em>****************************</em>
which translates into a pattern like .*.*.*.*, which is very inefficient
to try to match, althought it is equivalent ot a single .*. At a certain
point preg was just giving up.

Since people actually do this, we should simplify the regex by treating
runs of * like a single *.

question/type/shortanswer/question.php

@@ -92,8 +92,10 @@ class qtype_shortanswer_question extends question_graded_by_strategy
         $pattern = self::safe_normalize($pattern);
         $string = self::safe_normalize($string);
 
-        // Break the string on non-escaped asterisks.
-        $bits = preg_split('/(?<!\\\\)\*/', $pattern);
+        // Break the string on non-escaped runs of asterisks.
+        // ** is equivalent to *, but people were doing that, and with many *s it breaks preg.
+        $bits = preg_split('/(?<!\\\\)\*+/', $pattern);
+
         // Escape regexp special characters in the bits.
         $escapedbits = array();
         foreach ($bits as $bit) {

question/type/shortanswer/tests/question_test.php

@@ -132,6 +132,12 @@ class qtype_shortanswer_question_test extends advanced_testcase {
                 '0', '*0*', false));
     }
 
+    public function test_compare_string_with_wildcard_many_stars() {
+        // Test the classic PHP problem case with '0'.
+        $this->assertTrue((bool)qtype_shortanswer_question::compare_string_with_wildcard(
+                '<em></em>', '***********************************<em>***********************************</em>', false));
+    }
+
     public function test_is_complete_response() {
         $question = test_question_maker::make_question('shortanswer');