check_is_number

/* isalnum example */
#include <stdio.h>
#include <ctype.h>
int main ()
{
  int i;
  char str[]="c3po...";
  i=0;
  while (!isdigit(str[i])) i++;
    printf ("The first %d characters are alphanumeric.\n",i);
  return 0;
}