CALCULATION_REGEXP_CELLREF is not sufficiently robust.
It treats some perfectly legal defined names, e.g. A1A, as cell refs.
When the Xlsx Writer tries to save a worksheet which uses such a name
in a formula in a cell, it throws an exception.
The new DefinedNameConfusedForCellTest is a simple demonstration.
The Regexp has been changed to ensure the name starts on a Word boundary,
and to make sure it is not followed by a word character or period.
This fixes the problem, and does not appear to cause any regression
problems in the test suite.
Closes#1263
Adrien Crivelli