iPhone how to check that a string is numeric only

static bool TextIsValidValue( NSString* newText, double &value )
{
    bool result = false;

    if ( [newText isMatchedByRegex:@"^(?:|0|[1-9]\\d*)(?:\\.\\d*)?$"] ) {
        result = true;
        value = [newText doubleValue];
    }
    return result;
}

- (IBAction) doTextChanged:(id)sender;
{
    double value;
    if ( TextIsValidValue( [i_pause stringValue], value ) ) {
        [i_pause setTextColor:[NSColor blackColor]];
        // do something with the value
    } else {
        [i_pause setTextColor:[NSColor redColor]];
    }
}