my $a = "x|xx|xx|xxxx|xx|xxx|xx|xx|x";
1 while ($a =~ s/\|xx\|/|x x|/);
print "[$a]\n";
__DATA__
[x|x x|x x|xxxx|x x|xxx|x x|x x|x]
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but as tachyon mentioned: what to do with the edge cases? (this method is not optimal by any means.)

-enlil