Take the string:

''a''' ''b'' zz'''c''

Here the bold and italic are overlapping. The Bold part is:

''' ''b'' zz'''

The Italic parts are:

''a''' '' and '' zz'''c''

The problem here is that while replacing the first, the internal shadow variable of the last Italic part is no longer matching the actual string. The zz gives just enough offset, that the match on the last Italic object now returns None, instead of the actual match.

This can be solved by first replacing the closing tag of the Bold with something that is of equal size than the closing string, so: three characters. That way the shadow of the Italic entries remain correct, and replacement works out fine.

In all of the cases we have seen this so far, it was always user-error for having unbalanced ', but that shouldn't cause a crash, ofc.