Pattern-based replacement UDF in TSQL

Pattern-based replacement UDF

Originally posted here.

As a personal challenge, I decided to write a UDF that will work just like
T-SQL's REPLACE() function, but using patterns as input.

The first question: How does REPLACE() handle overlapping patterns?

SELECT REPLACE('babab', 'bab', 'c')

--------------------------------------------------
cab

(1 row(s) affected)


SELECT REPLACE('bababab', 'bab', 'c')

--------------------------------------------------
cac

(1 row(s) affected)

It appears that SQL Server parses the input string from left to right,
replacing the first instance of the replacement string, and then continues
parsing to the right.

Next question: How to do the replacement on a pattern? As it turns out, this
is somewhat trickier than I initially thought. A replacement requires a starting
point -- easy to find using PATINDEX -- and an end point. But there is no
function for finding the last character of a pattern. So you'll see that the UDF
loops character-by-character, testing PATINDEX, in order to find the end of the
match. This is useful for situations like:

SELECT dbo.PatternReplace('baaa', 'ba%', 'c')

-- We know that the match starts at character 1... but where does it end?

Anyway, enough background, here's the code:

CREATE FUNCTION dbo.PatternReplace
(
   @InputString VARCHAR(4000),
   @Pattern VARCHAR(100),
   @ReplaceText VARCHAR(4000)
)
RETURNS VARCHAR(4000)
AS
BEGIN
   DECLARE @Result VARCHAR(4000) SET @Result = ''
   -- First character in a match
   DECLARE @First INT
   -- Next character to start search on
   DECLARE @Next INT SET @Next = 1
   -- Length of the total string -- 8001 if @InputString is NULL
   DECLARE @Len INT SET @Len = COALESCE(LEN(@InputString), 8001)
   -- End of a pattern
   DECLARE @EndPattern INT

   WHILE (@Next <= @Len)
   BEGIN
      SET @First = PATINDEX('%' + @Pattern + '%', SUBSTRING(@InputString, @Next, @Len))
      IF COALESCE(@First, 0) = 0 --no match - return
      BEGIN
         SET @Result = @Result +
            CASE --return NULL, just like REPLACE, if inputs are NULL
               WHEN  @InputString IS NULL
                     OR @Pattern IS NULL
                     OR @ReplaceText IS NULL THEN NULL
               ELSE SUBSTRING(@InputString, @Next, @Len)
            END
         BREAK
      END
      ELSE
      BEGIN
         -- Concatenate characters before the match to the result
         SET @Result = @Result + SUBSTRING(@InputString, @Next, @First - 1)
         SET @Next = @Next + @First - 1

         SET @EndPattern = 1
         -- Find start of end pattern range
         WHILE PATINDEX(@Pattern, SUBSTRING(@InputString, @Next, @EndPattern)) = 0
            SET @EndPattern = @EndPattern + 1
         -- Find end of pattern range
         WHILE PATINDEX(@Pattern, SUBSTRING(@InputString, @Next, @EndPattern)) > 0
               AND @Len >= (@Next + @EndPattern - 1)
            SET @EndPattern = @EndPattern + 1

         --Either at the end of the pattern or @Next + @EndPattern = @Len
         SET @Result = @Result + @ReplaceText
         SET @Next = @Next + @EndPattern - 1
      END
   END
   RETURN(@Result)
END

... And here's how you run it, with some sample outputs showing that it does,
indeed, appear to work:

SELECT dbo.PatternReplace('babab', 'bab', 'c')
                                                  
--------------------------------------------------
cab

(1 row(s) affected)


SELECT dbo.PatternReplace('babab', 'b_b', 'c')
                                                  
--------------------------------------------------
cab

(1 row(s) affected)


SELECT dbo.PatternReplace('bababe', 'b%b', 'c')

                                                  
--------------------------------------------------
cabe

(1 row(s) affected)

Hopefully this will help someone, somewhere. I haven't found any use for it
yet :)

Thanks to Steve Kass for posting some single-character replacement code which
I based this UDF on.