[100000000]#\,###\,##\,##\,###;[100000]#\,##\,###;#,###

the above is a custom formatting, which tries to accomplish the following:

99999 or below needs to format as normal US digit grouping (99,999, 65,897,
etc)

100000-999999 needs to format as #,##,###, so 1,00,000 on one end and
9,99,999 on the high end

1000000-999999999 needs to format as #,##,##,###, so 1,00,00,000 on one end
and 9,99,99,999 on the other end

the numbers above 100000000 are then formatted in the same sequence. So if
there is a number 100,000,000,000,000, the last 7 digits are as above, in
2/2/3 grouping, and then the next set of 7 digits is the same 2/2/3 grouping
(1,00,00,000,00,00,000)

The custom format I have gets me part of the way, but not the higher numbers
(you can try and see).

the other thing I cannot do is get it to do negative as well. In other
words, I need negative and positive to both format the same way (with
parentheses as negative indicators, preferably).

If anyone can solve, greatly useful tool for me.

--
Boris