use YAPE::Regex::Explain;
print YAPE::Regex::Explain ->new(
    qr/\b((?<!bar)(?!bar)\S{3,})/
)->explain;
__END__
The regular expression:

(?-imsx:\b((?<!bar)(?!bar)\S{3,}))

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):
----------------------------------------------------------------------
  \b                       the boundary between a word char (\w) and
                           something that is not a word char
----------------------------------------------------------------------
  (                        group and capture to \1:
----------------------------------------------------------------------
    (?<!                     look behind to see if there is not:
----------------------------------------------------------------------
      bar                      'bar'
----------------------------------------------------------------------
    )                        end of look-behind
----------------------------------------------------------------------
    (?!                      look ahead to see if there is not:
----------------------------------------------------------------------
      bar                      'bar'
----------------------------------------------------------------------
    )                        end of look-ahead
----------------------------------------------------------------------
    \S{3,}                   non-whitespace (all but \n, \r, \t, \f,
                             and " ") (at least 3 times (matching the
                             most amount possible))
----------------------------------------------------------------------
  )                        end of \1
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------