[exim] Show regex capture group in logs?

Hi, I want to capture part of a regex match (capture group) to put in
logs. This example using subject header works but is there a better way?

warn set acl_c_temp = ${if match{$h_Subject:}{<regex with capture 
group>}{$1}{none}}
      !condition = ${if eq{$acl_c_temp}{none}}
      logwrite = Notice: subject match is: $acl_c_temp

This also risky because must be careful to make sure "none" could never
come from the regex match group

PS: is there better local variable to use?

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