A reminder to self on how to use regex in c

regex_example.c

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

int main(int argc, char *argv[]){
        regex_t regex;
        int reti;
        char msgbuf[100];
        size_t max_groups = 1;
        regmatch_t groups[max_groups];
        
        /* Compile regular expression */
        reti = regcomp(&regex, "{ $date : [[:digit:]]\\{3\\} }", 0);
        if( reti ){ fprintf(stderr, "Could not compile regex\n"); exit(1); }

        /* Execute regular expression */
        reti = regexec(&regex, argv[1], max_groups, groups, 0);
        if( !reti ){
                puts("Match");
                char m[groups[0].rm_eo - groups[0].rm_so + 1];
                puts("Allocated");
                m[groups[0].rm_eo] = 0;
                strncpy(m, argv[1]+groups[0].rm_so, groups[0].rm_eo - groups[0].rm_so); 
                printf("%s \n", argv[1]+groups[0].rm_so);
                printf("%s \n", m);
        }
        else if( reti == REG_NOMATCH ){
                puts("No match");
        }
        else{
                regerror(reti, &regex, msgbuf, sizeof(msgbuf));
                fprintf(stderr, "Regex match failed: %s\n", msgbuf);
                exit(1);
        }

        /* Free compiled regular expression if you want to use the regex_t again */
        regfree(&regex);

        return 0;
}

/* Regex example
 * $ gcc test_regex.c
 * $ ./a.out '{ $date : 123 }notreached'
    Match
    Allocated
    { $date : 123 }notreached
    { $date : 123 } 
* 
*/