Actually, the current filter syntax is not significantly easier to learn than
regex syntax. Add the fact that the description of the current syntax is
incomplete and the fact that there's tons of very good documentation for regex
(as well as true specifications, which the current filter syntax lack).
Further add the fact that regex is used elsewhere and many, many people
already know it, and those who don't already know it can use it elsewhere if
they learn it here, whereas the current filter syntax is used only in
*httrack.

To me all this sums up to a bottom line pointing to even completely replacing
the current syntax with regex.

The only thing that would become in any relevant way more "difficult" for the
occasional newbie is that instead of "+*.gif" they should write "+.*\.gif".
Learning that takes only one glance at one example, so it can hardly be called
difficult (and hence the "-marks above).