It currently parsed by byte, which is not a good thing in a UTF-8 system. See the discussion at the bottom of PR #4203. That turns out to not be the change which led to this problem; that would have been PR #4323. That change came about because using Composer/Pcre revealed bugs in several regular expressions used in Writer/Xls/Parser. This ticket comes about because more bugs were revealed in the same module. The problem is that the `advance` method needs to process formulas character by character, but is instead doing it byte by byte. It is changed to advance by characters, and tests for non-ASCII characters are added.
oleibman