[exim] How to see matched string?

When you has a ACL like:

warn condition = ${if match{$h_Subject:}{\N(foo(bar))\N}}

How can see the matched subgroups? I tried to read manual, so I think it
said $1 can be use in the success string, but in a condition the success
string must to be "yes" so what is workaround? I like to put into
logwrite.

Thank you kindly.