@ISDIGIT[string] : Returns 1 if string is entirely composed of decimal digits (0- 9); 0 otherwise.
See also: @ISALNUM, @ISALPHA, @ISASCII, @ISCNTRL, @ISPRINT, @ISPUNCT, @ISSPACE, @ISXDIGIT.
Examples:
echo %@isdigit[0]
1
echo %@isdigit[123.456]
0
echo %@isdigit[-123]
0