$ perl -Mre=debug -le " q/yabba dabba doo/ =~ /y(.)(.)\2\1/ "
Compiling REx "y(.)(.)\2\1"
Final program:
   1: EXACT <y> (3)
   3: OPEN1 (5)
   5:   REG_ANY (6)
   6: CLOSE1 (8)
   8: OPEN2 (10)
  10:   REG_ANY (11)
  11: CLOSE2 (13)
  13: REF2 (15)
  15: REF1 (17)
  17: END (0)
anchored "y" at 0 (checking anchored) minlen 3
Guessing start of match in sv for REx "y(.)(.)\2\1" against "yabba dabba doo"
Found anchored substr "y" at offset 0...
Guessed: match at offset 0
Matching REx "y(.)(.)\2\1" against "yabba dabba doo"
   0 <> <yabba dabb>         |  1:EXACT <y>(3)
   1 <y> <abba dabba>        |  3:OPEN1(5)
   1 <y> <abba dabba>        |  5:REG_ANY(6)
   2 <ya> <bba dabba >       |  6:CLOSE1(8)
   2 <ya> <bba dabba >       |  8:OPEN2(10)
   2 <ya> <bba dabba >       | 10:REG_ANY(11)
   3 <yab> <ba dabba d>      | 11:CLOSE2(13)
   3 <yab> <ba dabba d>      | 13:REF2(15)
   4 <yabb> <a dabba do>     | 15:REF1(17)
   5 <yabba> < dabba doo>    | 17:END(0)
Match successful!
Freeing REx: "y(.)(.)\2\1"

##</code><code>##

use YAPE::Regex::Explain;
print YAPE::Regex::Explain->new(
qr/y(.)(.)\2\1/
)->explain;
__END__
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
----------------------------------------------------------------------