how to get only numbers from a string in sql server

declare  @strAlphaNumeric nvarchar(max)
set @strAlphaNumeric='aa12bbc1234c'
DECLARE @intAlpha INT
SET @intAlpha = PATINDEX('%[^0-9]%', @strAlphaNumeric)
BEGIN
WHILE @intAlpha > 0
BEGIN
SET @strAlphaNumeric = STUFF(@strAlphaNumeric, @intAlpha, 1, '' )
SET @intAlpha = PATINDEX('%[^0-9]%', @strAlphaNumeric )
print @strAlphaNumeric+','
END
END
select ISNULL(@strAlphaNumeric,0)


CREATE FUNCTION dbo.udf_GetNumeric
(@strAlphaNumeric VARCHAR(256))
RETURNS VARCHAR(256)
AS
BEGIN
DECLARE @intAlpha INT
SET @intAlpha = PATINDEX('%[^0-9]%', @strAlphaNumeric)
BEGIN
WHILE @intAlpha > 0
BEGIN
SET @strAlphaNumeric = STUFF(@strAlphaNumeric, @intAlpha, 1, '' )
SET @intAlpha = PATINDEX('%[^0-9]%', @strAlphaNumeric )
END
END
RETURN ISNULL(@strAlphaNumeric,0)
END
GO

/* Run the UDF with different test values */
SELECT dbo.udf_GetNumeric('') AS 'EmptyString';
SELECT dbo.udf_GetNumeric('asdf1234a1s2d3f4@@@') AS 'asdf1234a1s2d3f4@@@';
SELECT dbo.udf_GetNumeric('123456') AS '123456';
SELECT dbo.udf_GetNumeric('asdf') AS 'asdf';
SELECT dbo.udf_GetNumeric(NULL) AS 'NULL';
GO