limiting regex scanning

I am new to using exiscan (having used exim for years) and would like to know if it 
possible to limit the amount of data scanned by regex.

I would like to detect base64 encoded encrypted ZIP files in a MIME ACL.

That means checking just the first few chracters of the base64 data.  If I use

  regex = ^UEsDB....[Q-Za-fw-z0-9\+/]

then all of each attachment will be scanned, with the corresponding CPU usage.

It would be nice to be able to restrict the regex to work on just the first dozen 
bytes of each attachment, which would be quick.

Can I do that?  If not, can that be put on a wish-list?

Phil.
---------------------------------------
Phil Chambers ([email protected])
University of Exeter