Search and replace in string

Question

How can i do search and replace in C? I'm trying to do function to replace html entities in string. I already have the function to find beginning and end of html entities but i can't figure out how to replace them.

Here is what i have already:

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

struct entity {
    char *entity;
    char *substitute;
};

void replacehtmlentities(char *str, char *dest) {
    int i;
    char *begin = NULL;
    char *end;

    struct entity entities[] = {
        { "&nbsp;", " " },
        { "&lt;", "<" },
        { "&gt;", ">" },
        { "&amp;", "&" },
        { "&euro;", "€" },
        { "&copy;", "©" },
        { "&reg;", "®" },
        { NULL, NULL },
    };

    for (i = 0; entities[i].entity; i++) {
        while (begin = strstr(str, entities[i].entity)) {
            end = begin + strlen(entities[i].entity);
            // how to replace
        }
    }
}

int main(int argc, char **argv) {
    char *str = "space &nbsp; lowerthan &lt; end";

    printf("%s\n", str);

    replacehtmlentities(str);

    printf("%s\n", str);

    return EXIT_SUCCESS;
}

Answer 1

The short answer is to use an existing string replacement function. There's one on my site at http://creativeandcritical.net/str-replace-c/ (the current version is named replace_str2). The changes you'd need to make to your code to use it (tested) are:

• Add #include <stddef.h> to the other includes.
• Copy the replace_str2 function into the file above the replacehtmlentities function.

• Change the prototype of the function replacehtmlentities to:

  char *replacehtmlentities(char *str)
  

• Add to that function the following variable declarations:

  char *tmp = NULL;
  char *tmp2 = str;
  

• Replace in that function the code:

      while (begin = strstr(str, entities[i].entity)) {
          end = begin + strlen(entities[i].entity);
          // how to replace
      }
  

with:

        tmp = replace_str2(tmp2, entities[i].entity, entities[i].substitute);
        if (i) free(tmp2);
        tmp2 = tmp;

• Add a final return to that function:

  return tmp2;
  

• In main, change your call of that function to:

  str = replacehtmlentities(str);
  

As an additional note: in main, str will now reference memory allocated with malloc. If/when you no longer require this string, you can free the memory by calling free(str).

Answer 2

The pointer str points to a string literal, and string literals are read only (i.e. constant). Trying to modify a string literal will lead to undefined behavior.

The solution is very simple: Declare str as an array:

char str[] = "space   lowerthan < end";

However be careful when replacing sequences in a string, so you don't replace a shorter sub-string with a longer, as then you might write beyond the end of the string.