I have posted some of the User Defined Functions to get 
1.only Numeric, 
2.Alpha
3.AlphaNumeric values from the String

Function to get Only Numeric Values from the string

CREATE FUNCTION dbo.NumericOnly(
    @string VARCHAR(MAX)
)
RETURNS VARCHAR(MAX) AS
BEGIN
    DECLARE @intAlpha INT
    SET @intAlpha = PATINDEX('%[^0-9]%', @string)
    WHILE @intAlpha > 0
    BEGIN
        SET @string = STUFF(@string, @intAlpha, 1, '' )
        SET @intAlpha = PATINDEX('%[^0-9]%', @string)
    END
    RETURN     @string
END
GO
SELECT dbo.NumericOnly('abcd!@#ABCD#$%123%^%^') Output :  123

Function to get Only Alpha Values from the string 

CREATE FUNCTION dbo.AlphaOnly(
    @string VARCHAR(MAX)
)
RETURNS VARCHAR(MAX) AS
BEGIN
    DECLARE @intAlpha INT
    SET @intAlpha = PATINDEX('%[^a-zA-Z]%', @string)
    WHILE @intAlpha > 0
    BEGIN
        SET @string = STUFF(@string, @intAlpha, 1, '' )
        SET @intAlpha = PATINDEX('%[^a-zA-Z]%', @string)
    END
    RETURN     @string
END
GO
SELECT dbo.AlphaOnly('abcd!@#ABCD#$%123%^%^') Output :  abcdABCD

Function to get Only AlphaNumeric Values from the string 

CREATE FUNCTION dbo.AlphaNumericOnly(
    @string VARCHAR(MAX)
)
RETURNS VARCHAR(MAX) AS
BEGIN
    DECLARE @intAlpha INT
    SET @intAlpha = PATINDEX('%[^a-zA-Z0-9]%', @string)
    WHILE @intAlpha > 0
    BEGIN
        SET @string = STUFF(@string, @intAlpha, 1, '' )
        SET @intAlpha = PATINDEX('%[^a-zA-Z0-9]%', @string)
    END
    RETURN     @string
END
GO
SELECT dbo.AlphaNumericOnly('abcd!@#ABCD#$%123%^%^')  Output :  abcdABCD123