Unable to match regex in C - c

Question

I have a problem with following regex:
prefix:\w+,\w+,\s*-?[0-9]{1,4}\s*,\s*-?[0-9]{1,4}\s*,\s*-?[0-9]{1,4}\s*,(?:\w+)
The match string is the following:
prefix:string,string,-100,100,0,string
I cannot match this string in my C code. At least I get a successful match on some online tool where I generated this regex. Also there were compilation warnings about "\" characters in regex so I replaced it as explained in c-compiler-warning-unknown-escape-sequence-using-regex-for-c-program. Regex after compilation warning fixes:
prefix:\\w+,\\w+,\\s*-?[0-9]{1,4}\\s*,\\s*-?[0-9]{1,4}\\s*,\\s*-?[0-9]{1,4}\\s*,(?:\\w+)
Here's the testing code:
#include <stdio.h>
#include <regex.h>
#include <stdlib.h>
#define REGEX "prefix:\\w+,\\w+,\\s*-?[0-9]{1,4}\\s*,\\s*-?[0-9]{1,4}\\s*,\\s*-?[0-9]{1,4}\\s*,(?:\\w+)"
const char *input = "prefix:string,string,-100,100,0,string";
int main(){
int rc;
regex_t regex;
rc = regcomp(&regex, REGEX, 0);
if (rc != 0) {
fprintf(stderr, "Could not compile regex\n");
exit(1);
}
rc = regexec(&regex, input, 0, NULL, 0);
if (rc == 0) {
printf("Match!\n");
return 0;
}
else if (rc == REG_NOMATCH) {
printf("No match\n");
return -1;
}
else {
perror("Error\n");
exit(1);
}
return 0;
}
I use gcc version 6.2.0 20161005 (Ubuntu 6.2.0-5ubuntu12)

Answer 1

You need to do two things:
Use REG_EXTENDED flag to compile the regex (so that extended regular expression flavor (ERE) is enabled), else the limiting quantifier will need escaping, and there may be other potential issues
Remove the non-capturing group (?:...) as POSIX does not support this construct.
To make \w+ optional, just replace (?:\w+) with \w*.
Use
#define REGEX "prefix:\\w+,\\w+,\\s*-?[0-9]{1,4}\\s*,\\s*-?[0-9]{1,4}\\s*,\\s*-?[0-9]{1,4}\\s*,\\w*"
^^^
and then
rc = regcomp(&regex, REGEX, REG_EXTENDED);
^^^^^^^^^^^^
See the C demo.
Also, see more about the Extended Regular Expressions that are enabled by REG_EXTENDED, and POSIX Bracket Expressions is also a good resource to learn differences between BRE (Basic Regular Expressions) and ERE flavors.