gistfile1.c

#include <stdio.h>

size_t strlen(const char *__s); 
char* strstr(const char *__haystack, const char *__needle); 
char* strcpy(char* __dest, const char* __src);
char* strncpy(char* __dest, const char* __src, size_t __n);
void* malloc(size_t __size);
void* memcpy(void* __dest, const void* __src, size_t __n);
 
// This is supposed to be simple, no negative positions, etc.
char* substr(char* haystack, int start, int length) {
  if (length >= strlen(haystack) || start >= strlen(haystack) || start < 0) {
		return NULL;
	} else {
		char* haybale = malloc(length + 1);
		if (NULL == haybale) {
			return NULL;
		} else {
			memcpy(haybale, &(haystack[start]), length);
			return haybale;
		}
	}
}
 
char* str_replace(const char* haystack, const char* needle, const char* replace) {
	int matchCount, hayLen = strlen(haystack), needleLen = strlen(needle), repLen = strlen(replace);
	const char *hsPtr, *hbPtr;
	for (hsPtr = haystack; (hbPtr = strstr(hsPtr, needle)); hsPtr = hbPtr + needleLen) {
		matchCount++;
	}
	int diffLen = 0;
	if (repLen > needleLen) {
		diffLen += (repLen - needleLen);
	} else {
		diffLen -= (needleLen - repLen);
	}
	char* haybale = (char*)malloc(sizeof(char) * hayLen + diffLen);
	if (NULL != haybale) {
		char* hbbPtr = haybale;
		for (hsPtr = haystack; (hbPtr = strstr(hsPtr, needle)); hsPtr = hbPtr + needleLen) {
			int gapLen = hbPtr - hsPtr;
			strncpy(hbbPtr, hsPtr, gapLen);
			hbbPtr += gapLen;
			strncpy(hbbPtr, replace, repLen);
			hbbPtr += repLen;
		}
		strcpy(hbbPtr, hsPtr);
	}
	return haybale;
}
 
int main(int argc, char* argv[]) {
	char* test = "this is a &lt;test&gt;";
	test = str_replace(test, "&lt;", "<");
	if (NULL == test) {
		puts("Failed!");
		return 0;
	}
	test = str_replace(test, "&gt;", ">");
	if (NULL == test) {
		puts("Failed!");
		return 0;
	}
	puts(test);
	return 0;
}