$errmsg =~ /^((?:\[.+?\]){3})(.*)/;

##</code><code>##

$errmsg =~ /^(?:\[.+?\]){3}(.*)/;

##</code><code>##

$errmsg =~ /^        # Start of string
             (       # Start match $1
               (?:   # Start group (don't save backreference)
                 \[.+?\]  # Match set of []'s
               ){3}  # End group and repeat 3 times
              )      # End match $1
            (.*)     # Slurp up everything that's left
          /x;        # Done