Javascript, If input cointains characters not declared on a list show message

[corei7]

Hi, I need a script that shows a message If an input (html) contains characters that are not on the list. At the moment I have this script:

var iChars = "ABCDEFGHIJKLMNOPQRSTVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_";
for (var i = 0; i < document.formulario.usuario_nombre.value.length; i++) {
    if (iChars.indexOf(document.formulario.usuario_nombre.value.charAt(i)) != -1) {
        alert ("Your username has special characters. \nThese are not allowed.\n Please remove them and try again.");
		document.formulario.usuario_nombre.focus()
        return false;
    }
}

That script "reads" the input and if a character matches with the written in the "iChars" variable, it shows a message, okay i need the opposite.. If any character in the input doesn't match with "iChars", show a message.. Thanks !