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my %good = map +($_ => 1), qw{ pre strong };
my $string = "xxx<pre>xxx<www>xxx<strong>xxx";
while ($string =~ m{<(\w+)>}g) {
  warn "bad word '$1'" unless $good{$1};
}

print "ok\n" if $string =~ m{
  ^ (
    [^<]
  | < (?: pre | strong ) >
  )* $
}x;