Replace all substrings in a string

replace-all.c

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

char *replace_substring_in_string(char *str, char *from, char *to)
{
    char *result;
    unsigned int str_len, from_len, to_len;

    str_len  = strlen(str);
    from_len = strlen(from);
    to_len   = strlen(to);


    if (str_len + 1 - from_len <= 0)
    {
        return NULL;
    }

    int match_at = -1;
    for (unsigned int i = 0; i < str_len + 1 - from_len; i++)
    {
        char match = 1;
        for (unsigned int j = 0; j < from_len; j++)
        {
            if (str[i + j] != from[j])
            {
                match = 0;
                break;
            }
        }
        if (match == 0)
        {
            continue;
        }
        match_at = i;
        break;
    }

    if (match_at == -1)
    {
        return NULL;
    }

    result = malloc(str_len + to_len - from_len + 1);

    memcpy(result, str, match_at);
    memcpy(&result[match_at], to, to_len);
    memcpy(&result[match_at + to_len], &str[match_at + from_len], str_len - match_at - from_len);
    result[str_len + to_len - from_len] = '\0';


    return result;
}

char *replace_all_substrings_in_string(char *str, char *from, char *to)
{
    char *result;
    unsigned int str_len, from_len, to_len;

    str_len  = strlen(str);
    from_len = strlen(from);
    to_len   = strlen(to);


    if (str_len + 1 - from_len <= 0)
    {
        return NULL;
    }

    int num_matches = 0;
    for (unsigned int i = 0; i < str_len + 1 - from_len; i++)
    {
        char match = 1;
        for (unsigned int j = 0; j < from_len; j++)
        {
            if (str[i + j] != from[j])
            {
                match = 0;
                break;
            }
        }
        if (match == 0)
        {
            continue;
        }
        num_matches++;
    }

    if (num_matches == 0)
    {
        return NULL;
    }

    int matches_at[num_matches];

    unsigned int k = 0;
    for (unsigned int i = 0; i < str_len + 1 - from_len; i++)
    {
        char match = 1;
        for (unsigned int j = 0; j < from_len; j++)
        {
            if (str[i + j] != from[j])
            {
                match = 0;
                break;
            }
        }
        if (match == 0)
        {
            continue;
        }
        matches_at[k++] = i;
    }

    result = calloc(str_len + (to_len - from_len) * num_matches + 1, sizeof(char));
    int result_len = str_len + (to_len - from_len) * num_matches;

    unsigned int s = 0;
    for (unsigned int i = 0; i < num_matches; i++)
    {
        //memcpy(&result[s], &str[s], matches_at[i]);
        memcpy(&result[matches_at[i] + s], to, to_len);
        /*memcpy(
            &result[matches_at[i] + to_len + s],
            &str[matches_at[i] + from_len + s],
            str_len - matches_at[i] - from_len
        );*/
        s += to_len - from_len;
    }
    //result[s] = '\0';


    int offset = 0;
    for (unsigned int i = 0; i < result_len; i++)
    {
        if (result[i] == '\0')
        {
            result[i] = str[i - offset];
        }

        for (unsigned int j = offset; j < num_matches; j++)
        {
            if (i - offset == matches_at[j])
            {
                offset += to_len - from_len;
                break;
            }
        }

    }
    result[result_len] = '\0';

    return result;
}

int main()
{
    char *replace_me = "C is a programming language";
    char *replace_from = "is a";
    char *replace_to = "is the best";

    char *str = replace_all_substrings_in_string(replace_me, replace_from, replace_to);

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

    free(str);

    return 0;
}