Basic debugging checklist, Tip #9: Demystify regular expressions by installing and using the CPAN module YAPE::Regex::Explain

The regular expression:

(?-imsx:y(.)(.)\2\1)

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):
----------------------------------------------------------------------
  y                        'y'
----------------------------------------------------------------------
  (                        group and capture to \1:
----------------------------------------------------------------------
    .                        any character except \n
----------------------------------------------------------------------
  )                        end of \1
----------------------------------------------------------------------
  (                        group and capture to \2:
----------------------------------------------------------------------
    .                        any character except \n
----------------------------------------------------------------------
  )                        end of \2
----------------------------------------------------------------------
  \2                       what was matched by capture \2
----------------------------------------------------------------------
  \1                       what was matched by capture \1
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------
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