void main()
{
	char *buff = new char[ 1024 ];
	strcpy(buff,"0123eHell sdfjkahskgna;rn;sjsdjk%fds$%%SFS%AS%%ASafsf$%World%fdd&%world% World");
	buff = replace(buff,"%World%","%%%Hi%%%");
	buff = replace(buff,"0123","1234567890");
}
............................
int find(const char* buff, const char* what)
{
	int j =  0 ;
	int i =  0 ;
	int len = strlen(buff);
	while (i < len)
	{
		if( buff[i] == what[j] )
		{	
			j++;
		
			if(j == strlen(what))
				return i-j+ 1 ;
		}
		else
			if(j)
			{
				i--;
				j= 0 ;
			}

		i++;
	}
	return - 1 ;
}

char* replace(char* buff, const char* from, const char* to)
{
	int iHead =  0 ;
	int jTail =  0 ;
	
	int Head_Len =  0 ;
	int Tail_Len =  0 ;
	int Len =  0 ;
	
	char *Head = NULL;
	char *Tail = NULL;

	if ((iHead = find(buff, from)) >=  0 )
	{
		jTail = iHead + strlen(from);
		
		//Выделяем хвост
		Tail_Len = strlen(&buff[jTail]);
		Tail = new char[Tail_Len+ 1 ];
		strcpy(Tail,&buff[jTail]);
		buff[iHead] = '\0';
		
		//выделяем голову
		Head_Len = strlen(buff);
		Head = new char[Head_Len+ 1 ];
		strcpy(Head,buff);
		
		//удаляем буфер
		delete [] buff;
		
		Len = Head_Len + strlen(to) + Tail_Len;
		buff = new char[Len];
		strcpy(buff,Head);
		strcat(buff,to);
		strcat(buff,Tail);

		delete Head;
		delete Tail;
	}	
	return buff;
}