fnmatch usage with complicate pattern C

Question

I'm only able to match simple patterns like: "[0-9]" with fnmatch("[0-9]", tocheck, 0).

If I try something more complicated with ? or . or even a combination of these how do I use fnmatch? I saw there are some flags that can do the trick, but I don't know how to use because I'm fairly new to C.

EDIT: I saw the comment asking to give more details:

#include <stdio.h>
#include <fnmatch.h>

int main(int argc, char **argv) {
    const char *patternOne = "[0-9]";
    const char *patternTwo = ".?[a-z0-9]*?*[a-z0-9]";
    int res = fnmatch(patternTwo, "0", 0);
    printf("Result: %d\n",  res);
}

If I use patternOne the result is 0 and if I change the string to match, the result change correctly. However if I use patternTwo I never get the 0 result for whatever string I pass to fnmatch. I need to match something like this in my program. It is for an university exam, so the patterns are very intricate.

Answer 1

#define _GNU_SOURCE
#include <stdio.h>
#include <fnmatch.h>

int main(int argc, char **argv) {
    const char *patternOne = "[0-9]";
    const char *patternTwo = ".?[a-z0-9]*?*[a-z0-9]";
    int res = fnmatch(patternTwo, "0", FNM_EXTMATCH);
    printf("Result: %d\n",  res);
}

Set FNM_EXTMATCH flag can use like .?[a-z0-9]*?*[a-z0-9] with pattern

FNM_EXTMATCH Besides the normal patterns, also recognize the extended patterns introduced in ksh. The patterns are written in the form explained in the following table where pattern-list is a | separated list of patterns.

?(pattern-list) The pattern matches if zero or one occurrences of any of the patterns in the pattern-list allow matching the input string.

(pattern-list) The pattern matches if zero or more occurrences of any of the patterns in the pattern-list allow matching the input string.

+(pattern-list) The pattern matches if one or more occurrences of any of the patterns in the pattern-list allow matching the input string.

@(pattern-list) The pattern matches if exactly one occurrence of any of the patterns in the pattern-list allows matching the input string.

!(pattern-list) The pattern matches if the input string cannot be matched with any of the patterns in the pattern-list.

Answer 2

Treat the pattern as a shell glob pattern. Given:

const char* patternTwo = ".?[a-z0-9]*?*[a-z0-9]";

There is no way "0" will match that. An example of a string that will match it would be: ".XaX9"

. matches .
X matches ?
a matches [a-z0-9]
X matches *?*
9 matches [a-z0-9]

The reason fnmatch() is different from glob() is so that the pattern "*" (which as a normal glob would match any string) will fail to match a file named ".profile" because dot files are treated as hidden (it is function designed to perform a file name match).