c:\@Work\Perl\monks>perl -wMstrict -le
"my %rx = ( 2 => 'Oops...' );
 my $rx = qr{ \b foo \b }xms;
 ;;
 my $n = 2;
 my $ry = qr{ $rx{2} X $rx{$n} Y (?:$rx){$n} }xms;
 print $ry;
"
(?msx-i: (?msx-i: \b foo \b ){2} X Oops... Y (?:(?msx-i: \b foo \b )){2} )

##</code><code>##

c:\@Work\Perl\monks>perl -wMstrict -le
"use Regexp::Common qw(net);
 ;;
 my $ipv4_A = qr{    $RE{net}{IPv4}    }xms;
 my $ipv4_B = qr{ \b $RE{net}{IPv4} \b }xms;
 ;;
 print 'match A' if '99999.9.9.99999' =~ $ipv4_A;
 print 'match B' if '99999.9.9.99999' =~ $ipv4_B;
"
match A

##</code><code>##

my $octet = qr{ \d+ }xms;
my $ipv4  = qr{ \b $octet (?: [.] $octet){3} \b }xms;