regex for http header in C

Note you do not need to escape a forward slash in a C regex library since the regcomp does not support regex delimiters.

All you need is to properly initialize the regmatch_t, size_t variables, use double escapes with the \s shorthand character class, and pass the REG_EXTENDED flag to the regex compiler.

I also suggest reducing the pattern to just 3 capture groups:

const char *str_regex = "([A-Za-z]+) +(http?://.*) +(HTTP/[0-9][.][0-9])";

Note the dot is "escaped" by putting it into a bracket expression.

Full C demo extracting GET, http://www.example.com and HTTP/1.1:

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

int main (void)
{
  int match;
  int err;
  regex_t preg;
  regmatch_t pmatch[4]; // We have 3 capturing groups + the whole match group
  size_t nmatch = 4; // Same as above
  const char *str_request = "GET http://www.example.com HTTP/1.1";

  const char *str_regex = "([A-Za-z]+) +(http?://.*) +(HTTP/[0-9][.][0-9])";
  err = regcomp(&preg, str_regex, REG_EXTENDED);
  if (err == 0)
    {
      match = regexec(&preg, str_request, nmatch, pmatch, 0);
      nmatch = preg.re_nsub;
      regfree(&preg);
      if (match == 0)
        {
          printf("\"%.*s\"\n", pmatch[1].rm_eo - pmatch[1].rm_so, &str_request[pmatch[1].rm_so]);
          printf("\"%.*s\"\n", pmatch[2].rm_eo - pmatch[2].rm_so, &str_request[pmatch[2].rm_so]);
          printf("\"%.*s\"\n", pmatch[3].rm_eo - pmatch[3].rm_so, &str_request[pmatch[3].rm_so]);
        }
      else if (match == REG_NOMATCH)
        {
          printf("unmatch\n");
        }
    }
  return 0;
 }