sub interpolate_nl {
	my $str = shift;
	$str =~ s/(?<!\\)(?:\\\\)*\K\\n/\n/g;
	$str =~ s/\\\\/\\/g;
	return $str;
	# on 5.14 and above, the above can be compacted to:
	#shift =~ s/(?<!\\)(?:\\\\)*\K\\n/\n/gr =~ s/\\\\/\\/gr
}

use Test::More tests=>9;
is interpolate_nl("a\\b"),          "a\\b";    # *
is interpolate_nl("a\\nb"),         "a\nb";
is interpolate_nl("a\\\\b"),        "a\\b";
is interpolate_nl("a\\\\nb"),       "a\\nb";
is interpolate_nl("a\\\\\\b"),      "a\\\\b";  # *
is interpolate_nl("a\\\\\\nb"),     "a\\\nb";
is interpolate_nl("a\\\\\\\\b"),    "a\\\\b";
is interpolate_nl("a\\\\\\\\nb"),   "a\\\\nb";
is interpolate_nl("a\\\\\\\\\\nb"), "a\\\\\nb";
# * \b and all other escapes are unhandled and ignored!