Check isalphanumeric in sql server

ALTER FUNCTION dbo.ISALPHANUMERIC
(
 @columnfield VARCHAR(5000)
)
RETURNS VARCHAR(5000)
AS
    BEGIN
        DECLARE @splcharposition INT
        SET @splcharposition = PATINDEX('%[^0-9A-Za-z]%', @columnfield)
    WHILE @splcharposition > 0
        BEGIN
            SET @columnfield = STUFF(@columnfield, @splcharposition, 1, '')
            SET @splcharposition = PATINDEX('%[^0-9A-Za-z]%', @columnfield)
        END
        SET @columnfield = @columnfield
    RETURN @columnfield
    END
GO