diff --git a/resources/index.html b/resources/index.html
index 090b0f7..f10904a 100644
--- a/resources/index.html
+++ b/resources/index.html
@@ -35,7 +35,7 @@
                 <div class=" pure-control-group">
                     <label for="shortUrl">Short URL (optional)</label>
                     <input type="text" name="shortUrl" id="shortUrl" placeholder="Only a-z, 0-9, - and _ are allowed"
-                        pattern="[A-Za-z0-9_-]+" />
+                        pattern="[a-z0-9\-_]+" title="Only a-z, 0-9, - and _ are allowed"/>
                 </div>
                 <div class="pure-controls">
                     <button class="pure-button pure-button-primary">Shorten!</button>
@@ -79,4 +79,4 @@
 
 </body>
 
-</html>
\ No newline at end of file
+</html>