[exim] How to check size or content of text part only?

Hi I wanted to add a test in exim acl config to catch current
hotmail&gmail spam when body only has something like:

fypaz uin

but $message_body has boundary parts and content-type headers and the
encoded pdf attachment. Howcan we do regular expr. for message text part
only. I not sure if message has plaintext with html or not but I think
just plaintext. Alternate, how can we test $message_size only for text
part?