$ cat test.cf
Empty2. E ::= Empty;

token Empty [""];

This produces:

• Haskell: Alex: crash: Empty language crashes alex with (Array.!): undefined array element haskell/alex#164
• Ocaml: ocamllex: File "LexTest.mll", line 51, character 0: syntax error.
• C/C++: flex: test.l:52: unrecognized rule
• Java/ANTLR: test/testLexer.g4:12:8: string literals and sets cannot be empty: []

So there should be a check for the empty regular expression, like for the nullable one.