Open A File Using Last String?

On my program, the user inputs a "name", "lastname" and "ID number" and saves the file, so the file will save as follows:

namelastname idnumber.txt (with nane/last name together).

My search feature asks for the name and last name and id no for the file and it will use that input to open the file.

But now i want a search feature that just asks for the id number. So how i do it so it reads the last string after the space (so after namelastname) of a text file, in this case, the ID number the user has inputted?

I thought of it theoretically:

the user inputs an id number in search, it records the length of that input (say 5 characters) and just reads the last 5 characters of the file name in directory X (that being, in this case, the ID number).