CREATE FUNCTION [dbo].[fn_StripCharacters]
(   @String          NVARCHAR(MAX),
    @MatchExpression VARCHAR(255)
)
RETURNS NVARCHAR(MAX)
AS
BEGIN
    SET @MatchExpression = '%['+@MatchExpression+']%'
    WHILE PatIndex(@MatchExpression, @String) > 0
        SET @String = STUFF(@String, PatIndex(@MatchExpression,@String), 1, '')
    RETURN @String
END

SELECT dbo.fn_StripCharacters('a1$%s#22d&39#f4$', '^a-z') as OnlyAlphaChars