Provisioning code regular expression - c

Question

I'm trying to develop a C code that checks the validity of "Provisioning code" string using regular expressions.
A "provisioning code" format should respect the following law:
If not an empty string, this argument SHOULD be in the form of a hierarchical descriptor with one or more nodes specified. Each node in the hierarchy is represented as a 4-character sub-string, containing only numerals or upper-case letters. If there is more than one node indicated, each node is separated by a "." (dot). Examples: "TLCO" or "TLCO.GRP2".
I started development using the code in this link http://web.archive.org/web/20160308115653/http://peope.net/old/regex.html
#include <sys/types.h>
#include <regex.h>
#include <stdio.h>
int main(int argc, char *argv[]){
regex_t regex;
int reti;
char msgbuf[100];
/* Compile regular expression */
reti = regcomp(&regex, "^a[[:alnum:]]", 0);
if( reti ){ fprintf(stderr, "Could not compile regex\n"); exit(1); }
/* Execute regular expression */
reti = regexec(&regex, "abc", 0, NULL, 0);
if( !reti ){
puts("Match");
}
else if( reti == REG_NOMATCH ){
puts("No match");
}
else{
regerror(reti, &regex, msgbuf, sizeof(msgbuf));
fprintf(stderr, "Regex match failed: %s\n", msgbuf);
exit(1);
}
/* Free compiled regular expression if you want to use the regex_t again */
regfree(&regex);
return 0;
}
this code works fine but my problem is what's the best regular expression that should be input of the function regcomp.
I started to try with a regular expression that matches with string that contains exactly 4 characters uppercases or numerals that means example like TLCO or TLC2 trying with the regular expression "[A-Z0-9]{4}" but I get "No match" as output with matches examples like TLC2.
Is there a suggestion about the right regular expression that should be input of regcomp and matches with "provisioning code"?

Answer 1

You may use the following regex that will work alright if you also pass REG_EXTENDED flag to the regcomp method (for the $ and {m,n} modifier to work correctly):
^[A-Z0-9]{4}([.][A-Z0-9]{4})*$
C code:
reti = regcomp(&regex, "^[A-Z0-9]{4}([.][A-Z0-9]{4})*$", REG_EXTENDED);
Details
^ - start of string
[A-Z0-9]{4} - 4 uppercase ASCII letters or digits
([.][A-Z0-9]{4})* - zero or more sequences of:
[.] - a literal . char
[A-Z0-9]{4} - 4 uppercase ASCII letters or digits
$ - end of string.