if ($myvar =~ /^[^A-VYZ]*(X+[^A-VYZ]*)+$/) {
   print "Error\n";
}
[download]

This should let you match things like 9XX35X7 and still reject AXXBXC.

Update: It's really too bad that I don't know the alphabet, and I inadvertently left W out. ww pointed it out to me, and I didn't intentionally omit him!

...roboticus

When your only tool is a hammer, all problems look like your thumb.