[3.1b] Invalid regular expression in helo_access

Jun 17 10:05:04 postfix/smtpd[21553]: warning: regexp map /etc/postfix/helo_access, line 19: Invalid preceding regular expression

The only uncommented line is

/^{myhostname}$/  REJECT

changing it to

/^{$myhostname}$/  REJECT

seems to fix the warning