IF EXISTS (SELECT 1 FROM SYSOBJECTS WHERE name = 'fn_subword')
BEGIN
    DROP FUNCTION fn_subword
END
GO
CREATE FUNCTION fn_subword(@memo VARCHAR(1000))
RETURNS VARCHAR(1000)
AS
BEGIN
DECLARE @REVERSE VARCHAR(1000)= REVERSE(@memo)
DECLARE @Submemo VARCHAR(1000)
SET @Submemo = SUBSTRING(@REVERSE,CHARINDEX('.',@REVERSE) + 1,CHARINDEX('_',@REVERSE) - CHARINDEX('.',@REVERSE) - 1)

IF ISDATE(REVERSE(@Submemo)) = 1
BEGIN
    SET @REVERSE = REPLACE(@REVERSE,@Submemo + '_','')
    SET @Submemo = SUBSTRING(@REVERSE,CHARINDEX('.',@REVERSE) + 1,CHARINDEX('_',@REVERSE) - CHARINDEX('.',@REVERSE) - 1)
END


RETURN REVERSE(@Submemo)

END 
GO
DECLARE @Str VARCHAR(1000) = 'G0858号_中华人民共和国_中央人民政府（2009）16号_对外贸易会签联系单_2009-5-27.pdf'

SELECT dbo.fn_subword(@Str)