[root@devel3 root]# cat test.c
#include <stdio.h>
#include <regex.h>

int
main(int argc, char *argv[])
{
  int i;
  char *pat = "^[A-Z]+$";
  printf("Got %d args\n", argc-1 ); /* first arg prog name */
  for( i=1; i<argc; i++ ) {
      printf("Got %s\n",argv[i]);
      if ( match( argv[i], pat ) ) {
          printf("-->Match %s\n", pat);
      }
      else {
          printf("***Fail\n");
      }
  }
  return 0;
}

int
match(const char *string, char *pattern)
{
    int    status;
    regex_t    re;

    if (regcomp(&re, pattern, REG_EXTENDED|REG_NOSUB) != 0) {
        return(0);      /* report error */
    }
    status = regexec(&re, string, (size_t) 0, NULL, 0);
    regfree(&re);
    if (status != 0) {
        return(0);      /* report error */
    }
    return(1);
}

[root@devel3 root]# gcc test.c
[root@devel3 root]# ./a.out 123 ABC abc ABC123
Got 4 args
Got 123
***Fail
Got ABC
-->Match ^[A-Z]+$
Got abc
***Fail
Got ABC123
***Fail
[root@devel3 root]#