$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;