replace string in C

str_replace.c

// fork from https://github.com/irl/la-cucina/blob/master/str_replace.c
char* str_replace(char* string, const char* substr, const char* replacement) {
	char* tok = NULL;
	char* newstr = NULL;
	char* oldstr = NULL;
	int   oldstr_len = 0;
	int   substr_len = 0;
	int   replacement_len = 0;

	newstr = strdup(string);
	substr_len = strlen(substr);
	replacement_len = strlen(replacement);

	if (substr == NULL || replacement == NULL) {
		return newstr;
	}

	while ((tok = strstr(newstr, substr))) {
		oldstr = newstr;
		oldstr_len = strlen(oldstr);
		newstr = (char*)malloc(sizeof(char) * (oldstr_len - substr_len + replacement_len + 1));

		if (newstr == NULL) {
			free(oldstr);
			return NULL;
		}

		memcpy(newstr, oldstr, tok - oldstr);
		memcpy(newstr + (tok - oldstr), replacement, replacement_len);
		memcpy(newstr + (tok - oldstr) + replacement_len, tok + substr_len, oldstr_len - substr_len - (tok - oldstr));
		memset(newstr + oldstr_len - substr_len + replacement_len, 0, 1);

		free(oldstr);
	}

	free(string);

	return newstr;
}

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