Not to distract from your feature request, but in the meantime you can use the following:
if ($regex($1,/^\d$/)) { a single digit }
if ($regex($1,/^\d+$/)) { one or more digits }
if ($regex($1,/^[0-9]$/)) { a single digit }
if ($regex($1,/^[0-9]+$/)) { one or more digits }
RegEx can help you be very specific about the nature of a string or value.
Thanks @Racoon, but Wims has already gave me $regex($1,^\d+$) regex (i still not have regex knowlegde yet!) but this operator is still be useful either that an custom identifier.