sql server function never gets executed. Was able to create it without any errors. Checked for infinite loops too. Please help

Below is the function:

create function dbo.CleanTheStringAdv (@theString as nvarchar(500), @CleanMode as nvarchar(500)) 
returns nvarchar(500)  
AS
begin  

--define variables
declare @strAlphaNumeric nvarchar(500)  
declare @i int
declare @strChar nvarchar(500)
declare @CleanedString nvarchar(500)

--initiate values
set @CleanedString =''  
set @theString = @theString + ''
SET @i = 1

--Determine if we are looking for numeric values only or numbers and letters

SET @strAlphaNumeric = case @CleanMode
--case @CleanMode
WHEN '1'
THEN  '0123456789'
WHEN '2'
THEN  '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ'
WHEN '3'
THEN  '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ|'
else 0

end 

--Search through chars in the string passed to the function

    while (@i <= Len(@theString))

begin
set @strChar = substring(@theString, @i, 1)  

--if the current char being reviewed is valid then add it to the new string     
If (charindex(@strChar, @strAlphaNumeric) <>0) 

begin
set @CleanedString = @CleanedString + @strChar
end
end

return @CleanedString
end

---------

when I run, select dbo.CleanTheStringAdv('100651|210110P3444470','1')

it took nearly 14 hours and didnt yet return any output.

when I run, select dbo.CleanTheStringAdv('100651|210110P3444470','3')

below error message is seen:

Msg 245, Level 16, State 1, Line 1
Conversion failed when converting the varchar value '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ|' to data type int.

Any help is highly appreciated........