my $a = "x|xx|xx|xxxx|xx|xxx|xx|xx|x";
$a =~ s/(?<=\|)xx(?=\|)/x x/g;
print "[$a]\n";
__DATA__
[x|x x|x x|xxxx|x x|xxx|x x|x x|x]

##</code><code>##

$a = 'xx|xx|xx';
# should this be (use example above):
[xx|x x|xx]
# or should it be
[x x|x x|x x]
# in which case you will need to add an extra regex because you can't have 
# variable width lookbacks. this regex just processes those edge cases
$a =~ s/^xx(?=\|)|(?<=\|)xx$/x x/g;
# combined with the first part gives you some real perl line noise
# if you want it all in a single regex
$a =~ s/^xx(?=\|)|(?<=\|)xx(?=\|)|(?<=\|)xx$/x x/g;