You can create your own User Defined Function for current and future use.
You will need a calculated field using a formula based on the STRIP function to remove all non-numeric characters from your extracted field and the LEN function to report how many characters are left. You may also need one or more of the TRIM functions to eliminate spaces before using LEN().
Check the embedded help entries for these functions - I think the examples provided are relevant for your needs. (They are in V10 Help, I think, from memory, that V9 Help is much the same.)
The resulting field (whether visible or hidden) should give you the filtering option you require.