(?!) explained:

From perlre: "A zero-width negative look-ahead assertion. For example /foo(?!bar)/ matches any occurrence of "foo" that isn't followed by "bar"." So in other words, (?!) is a negative lookahead assertion that "must not match nothing" (an impossibility).

YAPE::Regex::Explain describes it simply like this: 

perl -MYAPE::Regex::Explain -e "printYAPE::Regex::Explain->new( qr/(?!
+)/ )->explain();"

The regular expression:

(?-imsx:(?!))

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):
----------------------------------------------------------------------
  (?!)                     fail
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------

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Dave

In reply to Re^4: regex at word boundary by davido
in thread regex at word boundary by mikeraz

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