Hi,

What's unclear is whether this string is allowed to be embedded inside a longer string, although your example regexes seem to suggest that it's ok. Second, it would be good to know if multiple of these x.x.x sequences are allowed to be present in the source string? Should "ax1x2xbx34x56xc" return two strings, "x1x2x" and "x34x56x", or the single string "x1x2xbx34x56x"?

Here's my TIMTOWTDI solution:

print "$_ => ", extract($_)//'invalid', "\n"
    for qw/ xxx x.x.x x12x..x x123x...x x1x2x...x
    x123x.x.x x12x1x ax1x2xbx34x56xc /;

sub extract {
    my ($x) = shift=~/(x.*x.*x)/;
    return unless length($x)%2
        && substr($x,(length($x)-1)/2,1) eq 'x';
    return $x;
}

__END__

xxx => xxx
x.x.x => x.x.x
x12x..x => x12x..x
x123x...x => x123x...x
x1x2x...x => x1x2x...x
x123x.x.x => x123x.x.x
x12x1x => invalid
ax1x2xbx34x56xc => x1x2xbx34x56x
[download]

Hope this helps,
-- Hauke D