No idea why the regex is saved that way - did not yet use the regex option.
But regarding your issue, did ever thought to do it the other way with a lot of filters like the following:
Issue summary > does not contain > a
Issue summary > does not contain > b
... and so on, so basically 26 filters, 1 for every character and additional 10 when you also want to match the filters for numbers.
Regarding the regex issue, maybe @[deleted] has more experience, otherwise I would recommend to open a issue report over at github:
https://github.com/osTicket/osTicket-1.8/issues/new