What?
This is a quick note on finding the last occurrence of a string in a longer string. This has to be in Transact SQL for a SQL Server instance only and not filtered by other code.


Why?
I have a string such as the following (column positions added for demo purposes): 
   String1.String2.String3.String4
1   5   10   15   20   25   30 -> length = 31
I'd like to end up with just the last part of this, ie "String4". So I need to delimit based on the dot/period (.) and use substring in a sort of reversed form.

For argument's sake, I'm assigning this string to the variable "haystack".


How?
Perhaps we should determine the position of the last needle first (reverse the haystack string and find needle): 
DECLARE @Haystack VARCHAR(31);
SET @Haystack = 'String1.String2.String3.String4';
-- I want "String4" from Haystack

SET @Delimiter = '.';

PRINT @Haystack;
-- yields "String1.String2.String3.String4"


SELECT REVERSE( @Haystack );
-- yields "4gnirtS.3gnirtS.2gnirtS.1gnirtS"


SELECT CHARINDEX(@Delimiter, REVERSE( @Haystack ))
-- yields "8"


SELECT SUBSTRING(@Haystack, CHARINDEX(@Delimiter, REVERSE(@Haystack)), LEN(@Haystack))
-- yields ".String2.String3.String4"


Not quite right, as we got the last occurrence counting from the beginning rather than the end, so we still need to work out the starting point: 
SELECT  LEN(@Haystack) - CHARINDEX(@Delimiter, REVERSE(@Haystack))
-- yields "23"


SELECT SUBSTRING(@Haystack, LEN(@Haystack) - CHARINDEX(@Delimiter, REVERSE(@Haystack)), LEN(@Haystack))
-- yields "3.String4"


Almost there, counting backwards and forwards it seems the index is just two characters off (as I don't want to include the period): 
SELECT LEN(@Haystack) - CHARINDEX(@Delimiter, REVERSE(@Haystack)) + 2
-- yields "25"


SELECT SUBSTRING(@Haystack, LEN(@Haystack) - CHARINDEX(@Delimiter, REVERSE(@Haystack)) + 2, LEN(@Haystack))
-- yields "String4"  Yay!!!


Done!

Untested but in theory:
SELECT REVERSE( SUBSTRING( REVERSE( @Haystack ), 1, CHARINDEX( @Delimiter, REVERSE( @Haystack ) ) - LEN( @Delimiter ) ) )