Returns the substring of the input string between the specified character indexes.
The substring begins at the character specified by the 0-based start index. If an end index is specified, the substring ends at the character immediately before this end index. Thus, the length of the substring is endIndex-startIndex. If the end index is omitted, the substring goes to the end of the input string.
Parameter 1 - any string.
Parameter 2 - the 0-based start index specifying where the substring should start.
Parameter 3 (Optional) - the 0-based end index specifying where the substring should end.
Returns: a substring of the input string.