use YAPE::Regex::Explain;
print YAPE::Regex::Explain->new(
qr/'\.(\d+)\.I\./
)->explain;
__END__
The regular expression:

(?-imsx:'\.(\d+)\.I\.)

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):
----------------------------------------------------------------------
  '                        '\''
----------------------------------------------------------------------
  \.                       '.'
----------------------------------------------------------------------
  (                        group and capture to \1:
----------------------------------------------------------------------
    \d+                      digits (0-9) (1 or more times (matching
                             the most amount possible))
----------------------------------------------------------------------
  )                        end of \1
----------------------------------------------------------------------
  \.                       '.'
----------------------------------------------------------------------
  I                        'I'
----------------------------------------------------------------------
  \.                       '.'
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------