convert varchar to bigint function

I want to create a function that converts a string of characters to 开发者_如何学JAVAbigint. If the conversion is not possible the function should return null. I want the function to work for normal representation (example '10000') and mantissa-exponent representation ('1e1+10') Here is what I have written so far:

ALTER FUNCTION [dbo].[udf_get_bigint]    
(  
 @character varchar(100)  
)  
RETURNS bigint  

AS  
BEGIN  

    if ISNUMERIC(@character)=0 return null
    if LEN(ltrim(rtrim(@character)))>25 return null
    declare @nr numeric(25,4)
    if charindex('e',lower(@character))>0   
    begin
        declare @real real

        **set @nr=CONVERT(real,@character)**
        if  @nr not between convert(numeric(25),-9223372036854775808) and 
               convert(numeric(25),9223372036854775807) 
                return null
        set @real = convert(real, @nr)
        return convert(bigint,convert(numeric(25),@real))

    end
    else
        set @nr=CONVERT(numeric(25,4),@character)
        if  @nr between convert(numeric(25),-9223372036854775808) and 
          convert(numeric(25),9223372036854775807) return convert(bigint,@nr)
                 return null
END  

Now the only problem appears when I need to deal with overflows for mantissa exponent representation. The bolded conversion falls in case of overflow; but what I want it to do is to return null. How can I put some preconditions on that conversion so that it does not fall anymore.

call example : select dbo.udf_get_bigint('3e0210') output: Arithmetic overflow error converting expression to data type real.

Use float instead of real. It may contradict the variable name, but it makes that part of the script work

declare @real float

This code will verify

select CONVERT(float,'3e0210')