This function can be used to search a sub-string in a string in reverse order starting from the end of the string. The first argument is the string we are searching in. The second argument is the string that we actually want to find in the first argument. The third optional argument is the position where the search is to be started. If this argument is missing the search starts with the last character position of the string. The function returns the position where the sub-string can be found in the first string. If the searched sub-string is not found in the string then the return value is undef.