UDF Taking Too Long To Run Workaround ?

I have written a UDF into which I pass a table name, field name, value of the field, whether alpha characters are valid, whether numerics are valid, and a string of alphanumerics that are valid. I return back a string with all invalid characters removed. Unfortunately when I use this on names and addresses in an 12000 row table, it takes forever to run. Can anyone think of an easy way to do this which isn't so labour intensive. Please see code below.

NB CHAR(32) is space, CHAR(45) is -,CHAR(39) is '

CREATE FUNCTION dbo.UDF_RemoveInvalidCharacters
( @sTableName varchar(50),-- e.g. 'Contact'
@sFieldname varchar(50),-- e.g. 'Lastname'
@sFieldValue varchar(500),-- e.g. 'Jeremi@h O''Grady84'
@sAlphaValid char(1),-- e.g. 'Y'
@sNumericValid char(1),--e.g. 'N'
@sAlphanumericsValid varchar(500))--'CHAR(32):CHAR(45):CHAR(39)'
RETURNS varchar(500)
AS
BEGIN
DECLARE @sReturnValue varchar(500),
@nTableID int,
@nFieldLength int,
@nCurrentPos int,
@sTestChar char(1),
@sValid char(1),
@nAlphanumericPos int,
@sAlphanumericTest varchar(8),
@sTempTestChar varchar(8),
@sAlphasFound char(1),
@sNumericsFound char(1),
@sAlphanumericsFound char(1)

--Get ID of table that the field is on
SELECT @nTableID = [id]
FROM SYSOBJECTS
WHERE [name] = @sTableName

--Get the length of the field
SELECT @nFieldLength = sc.length
FROM SYSOBJECTS so, SYSCOLUMNS sc
WHERE so.id = @nTableID
AND sc.id = @nTableID
AND sc.name = @sFieldName

--Initialise values
SET @sReturnValue = ''
SET @nCurrentPos = 1
SET @sValid = 'N'
SET @sAlphasFound = 'N'
SET @sNumericsFound = 'N'
SET @sAlphanumericsFound = 'N'

--Test each character to ensure it is valid before adding it to the return string, a string consisting solely of alphanumeric characters would be wrong
WHILE @nFieldLength >= @nCurrentPos
BEGIN
SET @sTestChar = substring(@sFieldValue,@nCurrentPos,1)
IF @sAlphaValid = 'Y' --alphas are valid
BEGIN
IF UPPER(@sTestChar) in ('A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z')
BEGIN
SET @sValid = 'Y'
SET @sAlphasFound = 'Y'
END

END
IF @sNumericValid = 'Y' AND @sValid <> 'Y'--numerics are valid
BEGIN
IF @sTestChar in ('0','1','2','3','4','5','6','7','8','9')
BEGIN
SET @sValid = 'Y'
SET @sNumericsFound = 'Y'
END
END
SET @nAlphanumericPos = 1
WHILE LEN(@sAlphanumericsValid) > @nAlphanumericPos AND @sValid <> 'Y' --alphanumerics that are valid
BEGIN
IF CHARINDEX(':',SUBSTRING(@sAlphanumericsValid,@nAlphanumericPos,LEN(@sAlphanumericsValid))) > 0
BEGIN
SET @sAlphanumericTest = SUBSTRING(@sAlphanumericsValid,@nAlphanumericPos,CHARINDEX(':',SUBSTRING(@sAlphanumericsValid,@nAlphanumericPos,LEN(@sAlphanumericsValid)))-1)
END ELSE
BEGIN
SET @sAlphanumericTest = SUBSTRING(@sAlphanumericsValid,@nAlphanumericPos,(LEN(@sAlphanumericsValid)-@nAlphanumericPos)+1)
END
SET @sTempTestChar = 'CHAR(' + RTRIM(LTRIM(STR(ASCII(@sTestChar)))) + ')'
IF @sTempTestChar = @sAlphanumericTest AND (@sAlphasFound = 'Y' OR @sNumericsFound = 'Y') --alphanumerics are only valid once we have alpha or numerics
BEGIN
SET @sValid = 'Y'
SET @sAlphanumericsFound = 'Y'
END
SET @nAlphanumericPos = @nAlphanumericPos + LEN(@sAlphanumericTest) + 1
END
IF @sValid = 'Y'
BEGIN
SELECT @sReturnValue = @sReturnValue + @sTestChar
END
SET @nCurrentPos = @nCurrentPos + 1
SELECT @sValid = 'N'
END
IF @sAlphanumericsFound = 'Y' AND @sNumericsFound = 'N' AND @sAlphasFound = 'N' --alphanumerics on their own are not valid
BEGIN
SELECT @sReturnValue = ''
END
RETURN @sReturnValue --in the example I would get Jeremih O'Grady