You are trying with following standard C library function:
This function test only for decimal-digit character
http://www.freebsd.org/cgi/man.cgi?q...SD+6.2-RELEASE
This function test for hexadecimal-digit character
http://www.freebsd.org/cgi/man.cgi?q...SE&format=html
Best regards,
Iliyan Varshilov