Checking for invalid characters from an input with jQuerys

[ibnz]

I have an input box where the a username is input'd but if invalid characters are input'd, I want it to error. The code below is what I'm using; What would i put in the "something" section?\[code\] var numbers = new RegExp("SOMETHING"); $(this).removeClass("active"); if(($(this).val() == "") || $(this).val().match(numbers)) { $("#firstNameErrorMsg").html("First name can only contain letters. "); } else { $("#firstNameErrorMsg").html("OK"); }\[/code\]