added two string functions, doc updates

ml_string.cpp

@@ -77,3 +77,28 @@ std::string string_padd(const std::string &str, int pad_len, char fill_char, boo
 	else
 		return std::string(pad_len - str.size(), fill_char) + str;
 }
+
+
+std::string string_substring(const std::string & str, long pos, long count) {
+	size_t	start_pos = pos;
+
+	if (pos < 0) {
+		start_pos = str.size() - abs(pos);
+	}
+	
+	if ( (start_pos >= str.size()) || (count < 1) || (start_pos < 0) ) {
+		throw std::invalid_argument("Invalid parameter(s) for string-substr.");
+	}
+
+	return str.substr(start_pos, count);
+}
+
+size_t string_find_substr(const std::string & str, const std::string & pattern, long pos) {
+	if (pos >= str.size()) {
+		throw std::invalid_argument("Invalid parameter(s) for string-find.");
+	}
+
+	size_t p = str.find(pattern, pos);
+	
+	return p != str.npos ? p : -1;
+}