C regex validate filename under a folder - c

Question

I am new to regular expressions in C and I am trying to find if the given filename is under a folder using regex using regex.h library. This is what I have tried:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <regex.h>
int checkregex(char regex_str[100], char test[100]) {
regex_t regex;
printf("regex_str: %s\n\n", regex_str);
int reti = regcomp(&regex, regex_str, REG_EXTENDED | REG_ICASE);
if (reti) {
fprintf(stderr, "Could not compile regex\n");
exit(1);
}
reti = regexec(&regex, test, 0, NULL, REG_EXTENDED | REG_ICASE);
regfree(&regex);
return reti;
}
void main(int argc, char *argv[]) {
const char *safepath = "/home";
size_t spl = strlen(safepath);
char *fn = argv[1];
int noDoubleDots = checkregex("[^..\\/]", fn);
int allowedChars = checkregex("^[[:alnum:]\\/._ -]*$", fn);
int backslashWithSpace = checkregex(".*(\\ ).*", fn);
puts("noDoubleDots");
puts((noDoubleDots == 0 ? "Match\n" : "No Match\n"));
puts("allowedChars");
puts((allowedChars == 0 ? "Match\n" : "No Match\n"));
puts("backslashWithSpace");
puts((backslashWithSpace == 0 ? "Match\n" : "No Match\n"));
return;
}
My first attempt was just do not match if it includes .. (I couldn't even manage to do it) with noDubleDots. But then I tested and saw that file names and folder names can have .. in them, like folder..name/. So I wanted to exclude the ones with /.. or ../. But if the folder name is something like folder .. and it has another folder inside named folder2/ then the path will be folder\ ../folder2 and excluding ../ would result in wrong output.
In the code, allowedChars works fine. I think if I also checked if the file name has exactly .., \ .. or \ ([:alnum:])* to validate the file path, it would be done. But my regular expression doesn't seem to be working. For example, backslashWithSpace matches with asd / and asd\ /.
How can I check and make sure that the given path is under a folder using regular expressions? Thanks in advance.

Answer 1

POSIX offer a nice function realpath()
realpath() expands all symbolic links and resolves references to /./,
/../ and extra '/' characters in the null-terminated string named by
path to produce a canonicalized absolute pathname. The resulting
pathname is stored as a null-terminated string, up to a maximum of
PATH_MAX bytes, in the buffer pointed to by resolved_path. The
resulting path will have no symbolic link, /./ or /../ components.
If you can use it, I think it will fit your need, if not maybe you could copy the source code.