C regex validate filename under a folder

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.