I think you missed a sub keyword in there...

FILTER_ONLY regex => sub { s/\\s/[\\s\xA0]/g };
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Also, this wont always work, since perl cannot nest character classes. What about these (albeit contrived) tests?

print "\xA0" =~ /[A-Z\s]/ ? "Yay!\n" : "Hmmm\n";
print '[]'   !~ /[A-Z\s]/ ? "Yay!\n" : "Hmmm\n";
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-Blake