Wednesday, 27 April 2011

Dynamic repeatable character replacement

Sometime you need to eliminate duplicated char in string. Static replacement like
SET @Output_String=REPLACE(@teststring,'__','++')
is good for not big and predictable values. But what if you need to change replaced chars dynamically or its quantity is big enough?
In that case you can use dynamic replacement. Here is an example:

SET @Output_String=REPLACE(@teststring,'__','++')

DECLARE @Duplicate NVARCHAR(100)= '#$'
DECLARE @TestString NVARCHAR(MAX)= 'test_test__f##f2$$g'
DECLARE @Replacement NVARCHAR(MAX)= ''
DECLARE @OutputString NVARCHAR(MAX)= @teststring ;
WITH    numbers
          AS ( SELECT   ROW_NUMBER() OVER ( ORDER BY o.object_id, o2.object_id ) Number
               FROM     sys.objects o
                        CROSS JOIN sys.objects o2
          AS ( SELECT   SUBSTRING(@Duplicate, 1, 1) CHAR ,
                        CAST(1 AS INT) [LEVEL]
               UNION ALL
               SELECT   SUBSTRING(@Duplicate, numbers.Number, 1) CHAR ,
                        CAST(numbers.Number AS INT) [LEVEL]
               FROM     numbers
                        JOIN chars ON chars.Level + 1 = numbers.Number
               WHERE    LEN(SUBSTRING(@Duplicate, numbers.Number, 1)) > 0
          AS ( SELECT   REPLICATE(CHAR, numbers.number) Repl ,
               FROM     chars
                        CROSS JOIN numbers
    SELECT  @OutputString = REPLACE(@OutputString, Repl, @Replacement)
    FROM    replicated
    WHERE   number <= LEN(@TestString)

SELECT  @OutputString
You can modify @Duplicate string which is storage for replaced characters. Also @Replacements is a string which will be inserted instead of duplicated chars

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