If you want anything that isn't a digit or a leter (or an underscore), then you might consider using: \W

while ($in =~ /\W/) {
  # do something
}
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Or:

while ($in !~ /\w/) { ... }
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Otherwise:

while ($in =~ /[^a-zA-Z0-9]/) {
  # do something
}
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Or:

while ($in !~ /[a-zA-Z0-9]/) { ... }
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There's not much to chose between =~ and !~, but I generally like the emphasis of saying "as long as it's not one of these things..."