If 11.html is a "catch-all", then my current code is good.
Indeed, right now my code does the following: if the lower-cased version of a keyword starts with [a-z][a-z], then it appends that keyword (and its definition) to the corresponding file; otherwise, it appends it to 11.html.
argon -> ar.html
yoga -> yo.html
a- -> 11.html
-meter -> 11.html
o'clock -> 11.html
My previous doubt was raised by the fact that, in the official Italian dictionary, I found àa.html or wü.html files, suggesting that also accented characters are "allowed" to be there.
For confirming these issues, I will experiment a bit, but now it is quite late...