[exim] Regexp match on a header

I am trying to pick out the IP addresses from Received headers in the
DATA ACL. For now, only for logging, and to start I just want the first
one. Here is what I tried:

acl_check_data:

# Log the Received line IPs from upstream MTAs.

  warn
        set acl_m_data_rip = ${if match {$bh_received:}{\N[[](\d+[.]\d+[.]\d+[.]\d+)[]]} $1 {}}

  warn
        condition = $acl_m_data_rip
        logwrite = IP address from Received header is $acl_m_data_rip

And this was the result:

temporarily rejected after DATA: failed to expand ACL string \
"${if match {$bh_received:}{\N[[](\d+[.]\d+[.]\d+[.]\d+)[]]} $1 {}}": missing } at end of string

The braces are balanced, so why is a closing one "missing"?

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