How can I return pointer to the first regex occurrence in a string in C?

Question

At the moment, I am coding my own grep implementation and I have a problem with -o flag when it is used with regular expressions. The following code works properly for ordinary strings:

void do_o(char *pattern, char *line, flags options) {
  char *copy;
  char *temp;
  long unsigned int i;
  copy = line;
  if (options.v) {
    do_o_v(pattern, line, options);
  } 
  else {
      if (options.i) {
      while (copy && strcasestr(copy, pattern))
      {
        i = 0;
        temp = strcasestr(copy, pattern);
        while (i < strlen(pattern))
        {
          printf("%c", temp[i]);
          i++;
        }
        printf("\n");
        copy = strcasestr(copy, pattern) + strlen(pattern);
      }
    } else {
      while (copy && strstr(copy, pattern))
      {
        
        i = 0;
        temp = strstr(copy, pattern);
        while (i < strlen(pattern))
        {
          printf("%c", temp[i]);
          i++;
        }
        printf("\n");
        copy = strstr(copy, pattern) + strlen(pattern);
      }
    }
  }
}

However, I also need to do the same logic for with RegEx. So, everything I need is some function as strstr() or strcasestr() that will also support regex.

When I search regex examples and search my question on Google, I mainly come across examples with the functions regexec() and regcomp(). However the both return integer, not a pointer as I need. Any ideas how can i get a pointer of the first occurrence in the string?

P.S. I need pure C code

Answer 1

Any ideas how can i get a pointer of the first occurrence in the string?

regexec() has argument regmatch_t *pmatch, which is a pointer to a data structure containing the start and end index of a match. Using the index, you can find a pointer to the first occurrence.

Here is some example code, where get_first_match() returns a pointer to the first match in a string (warning: this does not implement any sort of error handling).

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

char *get_first_match(regex_t *pattern, char *str)
{
    char *ptr = NULL;
    regmatch_t match;
    if (regexec(pattern, str, 1, &match, 0) != REG_NOMATCH) {
        ptr = &str[match.rm_so];
    }

    return ptr;
}

int main()
{
    char *string = "Example1, Example2, Example3.";
    regex_t *pattern;

    regcomp(pattern, "Example2", 0);

    printf("%s\n", get_first_match(pattern, string));

    regfree(pattern);
}

This will print "Example2, Example3." Take a look at the man page for more information.