How to restrict string.atoi ? ? ? ?

[Tsang, Edward1]

Hi I have a funciton that converts number written in hex to numerical value.
I use code like:

string.atoi(inputString,16) to do it.

For example:

0X06

shall be converted to 6.

But I  want to restrict user from using 0A06, 0a08 etc; only allows the
decimal case or the hex case, ie. 6,06,006, 0X06, 0x06 shall all be allowed
..... 

How can I do that?

Thanks