jQuery Regular Expression

jQuery Regular Expression

So I wrote a custom method that restricts the username to specific alphanumeric characters and special characters. Everything works as intended except I want to make sure the username starts with a letter. Here is my code

  1. jQuery.validator.addMethod(
              "RestrictSpecialCharacters",
              function(value,element){
                var hNum= /[^a-zA-Z0-9._]/;
                var inp=jQuery.trim(element.value).toLowerCase();
               
                if(hNum.test(inp)) {   
                    return false;
                }
                    else return true;
                },
                "Characters allowed: [A-Z] [0-9] [.] [_]"    
            );











How can a make it such that a letter will have to be the first character in the username?