regexec is not excluding the character

Question

I'm trying to match words which doesn't have a letter 'd' in it , but the regexec is still matching the words with letter d

int main(void) {
    regex_t regex;
    char *str = "dabcd";
    char *pattern = "[^d]*";
    
    int ret;

    ret = regcomp(&regex, pattern, 0);
    if (ret == 0) {
        printf("regex compilation successfully\n");
    } else {
        printf("regex compilation unsuccessful\n");
    }

    ret = regexec(&regex, str, 0, NULL, 0);
    if (ret == 0) {
        printf("there is a match\n");
    } else {
        printf("there is no match : %d\n", ret);

    }

    return 0;
}

how to solve it ? Is there something wrong with my regex expression?

Answer 1

Your expression only checks that at some point in the string there's a sequence of zero or more non-d characters. If you want the whole thing to match:

^[^d]*$

Where that translates to at the start of the string, zero or more non-d characters followed by the end of string.

This will require altering your compilation code:

ret = regcomp(&regex, pattern, REG_EXTENDED);

Where it's noted in the documentation:

Use POSIX Extended Regular Expression syntax when interpreting regex. If not set, POSIX Basic Regular Expression syntax is used.

The man pages for BSD express it even better:

Compile modern ("extended") REs, rather than the obsolete ("basic") REs that are the default.

So use that mode by default.

Tip: For debugging regular expressions use an explainer tool like Regex101.