refactor: Make the flag validator marginally better

Author: Daniel Arthur Gallagher

index.html

@@ -39,7 +39,7 @@ <h2>A javascript regular expression editor</h2>
     <div id="main">
       <div id="expression">
         <h2>Regular Expression:</h2>
-        /<input name="expression">/<input name="option" type="text" pattern="gimuy" spellcheck="false" autocomplete="off" title="Set any flags you want to apply to your expression">
+        /<input name="expression">/<input name="option" type="text" pattern="(g|i|m|u|y){0,5}" spellcheck="false" autocomplete="off" title="Set any flags you want to apply to your expression">
       </div>
 
       <div id="test_strings">