Example: Substring Replacement (Different Lengths) in C

// Substring Replacement (Different Lengths)
#include <stdio.h>
#include <stdlib.h> // For malloc, free
#include <string.h> // For strlen, strstr, memcpy, strcpy

// Function to replace all occurrences of a substring
char* replace_substring(const char *original, const char *find, const char *replace) {
    char *result;
    char *ins;    // Pointer to current insertion point in `result`
    char *tmp;    // Current search position in `original`
    int len_find = strlen(find);
    int len_replace = strlen(replace);
    int len_original = strlen(original);
    int count;    // Number of matches
    int new_len = 0;

    // Step 1: Count occurrences of 'find' to calculate the new total length
    for (count = 0, ins = (char*)original; (tmp = strstr(ins, find)) != NULL; count++) {
        ins = tmp + len_find;
    }

    // Step 2: Calculate the new string length
    new_len = len_original + (len_replace - len_find) * count;

    // Step 3: Allocate memory for the new string (+1 for null terminator)
    result = (char*)malloc(new_len + 1);
    if (!result) return NULL; // Handle allocation failure

    // Step 4: Construct the new string
    ins = result;
    tmp = (char*)original;
    while (count--) { // Loop through each match
        char *match = strstr(tmp, find); // Find the next match
        int len_prefix = match - tmp;   // Length of the segment before the match

        // Copy prefix
        memcpy(ins, tmp, len_prefix);
        ins += len_prefix;

        // Copy replacement string
        memcpy(ins, replace, len_replace);
        ins += len_replace;

        // Move `tmp` past the current `find` occurrence
        tmp = match + len_find;
    }

    // Step 5: Copy the remaining part of the original string
    strcpy(ins, tmp);

    return result; // Return the newly allocated string
}

int main() {
    const char *str = "Hello world, hello C! This is a hello world example.";
    const char *find = "hello";
    const char *replace = "greetings"; // Longer replacement

    printf("Original string: %s\n", str);

    char *new_str = replace_substring(str, find, replace); // Step 1: Call replacement function

    if (new_str) {
        printf("String after replacement: %s\n", new_str);
        free(new_str); // Step 2: Free the dynamically allocated memory
    } else {
        printf("Memory allocation failed or no replacements made.\n");
    }

    const char *str2 = "aaabbbaaa";
    const char *find2 = "bbb";
    const char *replace2 = "c"; // Shorter replacement

    printf("Original string 2: %s\n", str2);
    char *new_str2 = replace_substring(str2, find2, replace2);
    if (new_str2) {
        printf("String 2 after replacement: %s\n", new_str2);
        free(new_str2);
    }

    return 0;
}