regex.h-reg-test.c

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

void redie(int eval, regex_t *re, char *msg) {
    char buf[1024];

    regerror(eval, re, buf, sizeof(buf));

    errx(1, "%s: %s", msg, buf);
}

int main(int argc, char *argv[])
{
    regex_t re;
    regmatch_t match[10];
    int eval;
    char *str = argv[1] ? argv[1] : " hello";

    eval = regcomp(&re, "^ *| *$", REG_EXTENDED);
    if (eval) {
	redie(eval, &re, "regcomp");
    }

    match[0].rm_so = 0;
    match[0].rm_eo = strlen(str);
    printf("---  [%lld,%lld] --- <%s>", match[0].rm_so, match[0].rm_eo, str);
    eval = regexec(&re, str, 10, match, REG_STARTEND);
    printf("---  %d [%lld,%lld]\n", eval, match[0].rm_so, match[0].rm_eo);
    if (eval) {
	redie(eval, &re, "regexec: 0");
    }

    match[0].rm_so = match[0].rm_eo;
    match[0].rm_eo = strlen(str);
    printf("---  [%lld,%lld] --- <%s>", match[0].rm_so, match[0].rm_eo, str);
    eval = regexec(&re, str, 10, match, REG_NOTBOL | REG_STARTEND);
    printf("---  %d [%lld,%lld]\n", eval, match[0].rm_so, match[0].rm_eo);
    if (eval) {
	redie(eval, &re, "regexec: 1");
    }

    regfree(&re);

    return 0;
}

On macOS the above outputs:

$ ./pgm " hello"
---  [0,6] --- < hello>---  0 [0,1]
---  [1,6] --- < hello>---  1 [1,6]
pgm: regexec: 1: regexec() failed to match