use strict;
use warnings;
use Test::More tests => 6;

is(munge('R'),   'text',  'R');
is(munge('R1'),  '1text', 'R1');
is(munge('R2'),  '2text', 'R2');
is(munge('R3'),  'text3', 'R3'); # this here is the trickiness
is(munge('R13'), 'R13',   'R13 unchanged');
is(munge('R4'),  'R4',    'R4 unchanged');

sub munge {
    my $str = shift;
    no warnings;
    $str =~ s/^R(?:([12])|(3))?$/${1}text$2/;
    $str;
}

##</code><code>##

sub munge {
    my $str = shift;
    $str =~ s{^R(?:([12])|(3))?$}
             {(defined $1 ? $1 : '') . 'text' . (defined $2 ? $2 : '')}e;
    $str;
}