Not only it is more clear, it also fixes a bug :)
```
  1) Failure:
TestLexer#test_regexp_escape_backslash_terminator [/home/whitequark/parser/test/test_lexer.rb:71]:
%r%blah\%blah%.
Expected: [:tREGEXP_BEG, "%r"]
  Actual: [:tREGEXP_BEG, "%r%"]
```