$ perl -le'
my $x = "AAAAAXXXXXXXAAAXXXXXAXXXXAAAA";
print $x;
$x =~ s/(?<=A)X+(?=A)/ "A" x ( $+[0] - $-[0] ) /eg;
print $x;
'
AAAAAXXXXXXXAAAXXXXXAXXXXAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAA

##</code><code>##

$ perl -le'
my $x = "AAAAAXXXXXXXAAAXXXXXAXXXXAAAA";
print $x;
$x =~ s/(?<=A)X+(?=A)/@{[ "A" x ( $+[0] - $-[0] ) ]}/g;
print $x;
'
AAAAAXXXXXXXAAAXXXXXAXXXXAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAA