One issue with changing "does not match" to "does not match regex" is that it could potentially confuse users into thinking that only a regular expression can be used with that rule. Instead, I recommend opening a feature request for the addition of a "does not match regex" option in the filter interface:
The "matches regex" option in the dropdown actually translates into just "matches" in the actual Exim filter itself. Nevertheless, Exim does seem to primarily expect a regex string for that option.
The option "does not match" in the dropdown translates into the same ("does not match") in the actual Exim filter. It appears to be very similar in nature, in that Exim primarily expects it too to contain a regex string.
In fact, the current label "does not match" in the dropdown could likely be causing greater confusion by leaving out the word "regex" because:
(a) The "matches regex" option in the dropdown includes the word "regex" to clarify its intended usage, but the companion option "does not match" doesn’t, causing inconsistency.
(b) The "equals" option in the dropdown translates into "is" in the actual Exim filter. However, its companion option "does not equal" (translating into "is not" in the Exim filter) appears to be completely missing from the dropdown. This leads some users (including myself) to mistake "does not match" for "does not equal".
With this in mind, let me know if you’d still like for me to open the specific feature request that you’ve suggested, or if you’d like different action to be taken.
A feature request is still a good idea for this particular topic as the existing filter operators in the drop-down menu have shown "does not match" for quite sometime. Feel free to post the feature request URL here once it's opened so others can vote for the request as they come across this thread.