Reverse only numerical parts of string in sql server

Question

With T-SQL, I'm trying to find the easiest way to reverse numbers in string. so for string like Test123Hello have Test321Hello.

[Before]           [After]
Test123Hello       Test321Hello
Tt143 Hello        Tt341 Hello
12Hll              21Hll
Tt123H3451end      Tt321H1543end

Answer 1

Just make use of PATINDEX for searching, append to the result string part by part:

CREATE FUNCTION [dbo].[fn_ReverseDigits]
(
    @Value nvarchar(max)   
)
RETURNS NVARCHAR(max)
AS
BEGIN

    IF @Value IS NULL
        RETURN NULL

    DECLARE 
        @TextIndex int = PATINDEX('%[^0-9]%', @Value), 
        @NumIndex int = PATINDEX('%[0-9]%', @Value), 
        @ResultValue nvarchar(max)  = ''

    WHILE LEN(@ResultValue) < LEN(@Value)
    BEGIN

        -- Set the index to end of the string if the index is 0
        SELECT @TextIndex = CASE WHEN @TextIndex = 0 THEN LEN(@Value) + 1 ELSE LEN(@ResultValue) + @TextIndex END
        SELECT @NumIndex = CASE WHEN @NumIndex = 0 THEN LEN(@Value) + 1 ELSE LEN(@ResultValue) + @NumIndex END

        IF @NumIndex < @TextIndex
            SELECT @ResultValue = @ResultValue + REVERSE(SUBSTRING(@Value, @NumIndex, @TextIndex -@NumIndex))
        ELSE
            SELECT @ResultValue = @ResultValue + (SUBSTRING(@Value, @TextIndex, @NumIndex - @TextIndex))

        -- Update index variables
        SELECT
            @TextIndex = PATINDEX('%[^0-9]%', SUBSTRING(@Value, LEN(@ResultValue) + 1, LEN(@Value) - LEN(@ResultValue))), 
            @NumIndex = PATINDEX('%[0-9]%', SUBSTRING(@Value, LEN(@ResultValue) + 1, LEN(@Value) - LEN(@ResultValue)))

    END


    RETURN @ResultValue
END

Test SQL

declare @Values table (Value varchar(20))
INSERT @Values VALUES
('Test123Hello'),
('Tt143 Hello'),
('12Hll'), 
('Tt123H3451end'),
(''),
(NULL)

SELECT Value, dbo.fn_ReverseDigits(Value) ReversedValue FROM @Values

Result

Value                ReversedValue
-------------------- --------------------
Test123Hello         Test321Hello
Tt143 Hello          Tt341 Hello
12Hll                21Hll
Tt123H3451end        Tt321H1543end

NULL                 NULL