Question

this is what I have so far:

$j(element)..keypress(function(e){
        var val = $j(this).val();
        if (("1234567890.").indexOf(e.charCode) > -1){
            if (val.length > 0){
                var regex = /(\+|-)?(\d*\.\d*)/;
                if (regex.test(val)){
                    return true;
                } else {
                    return false;
                }       
            } else {
                return true;
            }
        } else {
            return false;
        }
    });

but for some reason, the text field only allows one character… and not even a number

Answer 1

With the help of @elclanrs and @mgibsonbr, I came to this:

$j = jQuery.noConflict();
$j(function() {
    $j("input").keypress(function(e) {
        var val = $j(this).val();
        var regex = /^(\+|-)?(\d*\.?\d*)$/;
        if (regex.test(val + String.fromCharCode(e.charCode))) {
            return true;
        }
        return false;
    });
});

which can be tested here: http://jsfiddle.net/9U2Mw/5/