[exim] Regular expressions in .forward files?

Is there a way to use the following regex in .forward files with the
match operator?

if $rheader_from: matches "[^\x00-\x7f]\{5,\}"
or $rheader_subject: matches "[^\x00-\x7f]\{5,\}"

I get

rheader_from: matches [^ or $rheader_subject: matches [^
Filter error: error while compiling regular expression "[^": missing
terminating ] for character class at offset 2