use v5.20;

while (<DATA>)
{
  chomp;
  say "$_ => '$1' x '$2'" if /x(.*)x((??{ ".{".length($1)."}" }))x/;
}
__DATA__
xaxxax
xxx
x.x.x
x12x..x
x123x...x
x123x.x.x
##</code><code>##

xxx => '' x ''
x.x.x => '.' x '.'
x12x..x => '12' x '..'
x123x...x => '123' x '...'
x123x.x.x => '123' x '.x.'