Fixing validation of Twitter handler

Author: klakegg

src/main/java/no/java/submit/form/SpeakerForm.java

@@ -18,7 +18,7 @@ public class SpeakerForm {
     @Email(message = "Valid email address is required")
     public String email;
 
-    @Pattern(regexp = "^$|^@?(\\w\\-_){1,15}$", message = "Must be a valid Twitter username (e.g. '@username')")
+    @Pattern(regexp = "^$|^@?[\\w\\-]{1,15}$", message = "Must be a valid Twitter username (e.g. '@username')")
     public String twitter;
 
     @Pattern(regexp = "^$|^https:\\/\\/[a-z]{2,3}\\.linkedin\\.com\\/.*$", message = "Must be a valid LinkedIn address (e.g. 'https://www.linkedin.com/in/username')")