Incorrect syntax errors in sql server function. Please help.

This is my sql server function:

create function dbo.CleanTheStringAdv (@theString nvar

char(500), @CleanMode as int) 
returns nvarchar(500)  

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

return case @CleanMode

   WHEN 1
   THEN @strAlphaNumeric = '0123456789'
   WHEN 2
   THEN @strAlphaNumeric = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ'
   WHEN 3
   THEN @strAlphaNumeric = '|0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ'
   else 0

 end 

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

    while (@i <= Len(@theString))

begin

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

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

return @CleanedString

and below are the syntax errors I see when I press F5:

Msg 102, Level 15, State 1, Procedure CleanTheStringAdv, Line 24
Incorrect syntax near '='.
Msg 102, Level 15, State 1, Procedure CleanTheStringAdv, Line 39
Incorrect syntax near '@strChar'.
Msg 102, Level 15, State 1, Procedure CleanTheStringAdv, Line 43
Incorrect syntax near '@CleanedString'.

Any help is highly appreciated.....