Method slice string, substr, substring comparison

　　1 three process parameters are representative of start of the substring, the parameter represents the end position of the slice 2 and the substring, and represented in the substr is a length of the substring;
　　For negative attitude, when the parameter appears in position 1, slice substr and counted from the end, and the end of the counting does not support substring simply as 0; when the parameter appears in the second position, slice and the same process parameters substring 1: calculated from the end of the former, the latter is converted into 0, and substr depending on the length of the negative returns an empty string 0;
　　the case of less than 1 parameter 2 parameter, substring biggest difference is that it will then swap the two parameters taken substring, substr second parameter represented by a length and therefore no exception, slice Zeng substring search the whole story is still normal position, if it returns the starting position in the back end of the empty string.