use warnings;
use strict;

my @c = ("a { b } c ( d ) e", "a { b } c ( d ) e {{F}} ((G))");
print replace($_), "\n" for @c;

sub replace {
	my $d = shift;	
	my $nobrackets = qr/([^{}]+)/;
	$d =~ s/\{$nobrackets\}/leftbracket $1 rightbracket/g;
	return $d;
}