[exim] regexp comparison in Exim

Hello,

I want to change the subject of an email in a transport, but only if a
certain condition is met. With this condition I want to check wether the
subject contains a contain string. Unfortunately, the only regular
expression possiblities seem to be modifiers, not comparison operators.
But as I don't know the exact position of the string on the subject, I
would need to do something like this line of perl code:

if ($subject =~ /thisandthat/) {
    subject = "add something: $subject";
}

How could I do this?

Regards
Marten