#!/usr/bin/env perl -l

use strict;
use warnings;

my @test_strings = (
    'a { b } c ( d ) e',
    'a { b } c ( d ) e {{F}} ((G))'
);

my %replacement_for = (
    '{'   => ' lbracket ',
    ' { ' => ' lbracket ',
    '}'   => ' rbracket ',
    ' } ' => ' rbracket ',
);

for (@test_strings) {
    print "Initial string:  '$_'";
    replace();
    print "Replaced string: '$_'";
}

sub replace {
    s/( { | } |{(?!{)|(?<!})})/$replacement_for{$1}/g;
}