T-SQL Replace String

On a recent project I was tasked with writing a simple string replace function for T-SQL.  On the surface it seemed a fairly straightforward task using either PatIndex or charindex.  However, some of the specific requirements proved problematic.  There could be multiple replaces of the same text throughout the string, the replacement text may be the same as the text to find and the word may contain numbers and underscores. 

Using patindex in a loop seemed the best way forward to deal with the multiple replacements and the numbers and underscore characters.  The other problem was that I had to step through the original text to do the replacements so that if a text was replaced with the same text then the function wouldn't get stuck.  I achieved this by using sub string to get the remaining text after each replacement.

@OrigString varchar(8000), 
@LookFor varchar(1000), 
@ReplaceWith varchar(1000))

returns varchar(8000)
BEGIN
DECLARE @findIndex int
DECLARE @lengthLookFor int
DECLARE @lengthReplace int
DECLARE @totalLength int
DECLARE @tempString as varchar(8000)
DECLARE @counter int

SET @OrigString = '(' + @OrigString + ')'

SET @lengthLookFor = LEN(@LookFor)
SET @lengthReplace = LEN(@ReplaceWith)
SET @totalLength = LEN(@OrigString)
SET @tempString = @OrigString
SET @counter = 0

DECLARE @stuffindex int
SET @stuffindex = 0

DECLARE @substring int
SET @substring = 0

WHILE PATINDEX('%' + @LookFor +'[^a-z0-9]%', @tempString) != 0
BEGIN
 SET @findIndex = PATINDEX('%[^a-z0-9]' + @LookFor +'[^a-z0-9]%', @tempString) + 1
--SELECT @findIndex as 'findIndex'

 set @stuffindex =  @findIndex + @substring 
 if(@counter >0)
  set @stuffindex = @stuffindex - 1

 --SELECT @stuffindex as stuffindex

 SET @OrigString = STUFF(@OrigString, @stuffindex , @lengthLookFor, @ReplaceWith)
--SELECT @OrigString as 'outputstring'

SET @substring = @stuffindex + @lengthReplace
--SELECT @substring as substring

SET @tempString = SUBSTRING(@OrigString,@substring,@totalLength - @findIndex)
--SELECT @tempString as 'tempstring'

SET @counter = @counter + 1


END

SET @OrigString = SUBSTRING( @OrigString, 2, LEN(@OrigString)- 2)