That's a typo :). What diotalevi meant to write was /[^[:print:]]/ and it means (as per `perldoc perlre'):

C:\>perl -MYAPE::Regex::Explain -le"print YAPE::Regex::Explain->new(qr
+/[^[:print:]]/)->explain"
The regular expression:

(?-imsx:[^[:print:]])

matches as follows:

NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  [^[:print:]]             any character except: alphanumeric,
                           punctuation, and whitespace characters
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------
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