I am trying to find out whether a full word, say test, exists in a sentence and I’d like to do it in a case-insensitive manner. I took advantage of regex capabilities in C and used b in the pattern. Here is the program

#include <regex.h>
#include <stdio.h>
#include <string.h>

int main() {
    const char* hay = "This Is a TEst, or not!";

    regex_t regex;
    if (regcomp(&regex, "btestb", REG_EXTENDED | REG_ICASE)) {
        printf("compilation failedn");
    }

    regmatch_t match[1];
    if (regexec(&regex, hay, 1, match, 0) == REG_NOMATCH) {
        printf("couldn't matchn");
    }
}

When I run this program, it prints couldn't match but the sentence clearly contains the TEst word. Can someone point out the problem?