Neigher \K nor /r existed in Perl 5.6.x, which is the latest version of the regexp syntax that YAPE::Regex::Explain supports. (This is documented, and would have been pretty easy to test.)

perl -MYAPE::Regex::Explain -E 'say YAPE::Regex::Explain->new(q/@{[s".
+."."gr]}\K$"/)->explain'
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...produces the following output (assuming a Perl that is recent enough to understand \K and /r):

The regular expression:

(?-imsx:@{[s".."."gr]}\K$")

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):
----------------------------------------------------------------------
  @{                       '@{'
----------------------------------------------------------------------
  [s".."."gr]              any character of: 's', '"', '.', '.', '"',
                           '.', '"', 'g', 'r'
----------------------------------------------------------------------
  }                        '}'
----------------------------------------------------------------------
  \K                       'K'
----------------------------------------------------------------------
  $                        before an optional \n, and the end of the
                           string
----------------------------------------------------------------------
  "                        '"'
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------
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...which obviously is totally wrong. It completely misses that @{[.....]} interpolates code into a string (it instead thinks it's looking at a character class), it mistakes \K for literal "K", and mistakes $" for an end of line/string metacharacter, followed by a literal ", when it really should see the Perl special variable. Add to that the fact that it couldn't possibly understand the /r semantics, and the fact that we're using the regexp engine re-entrantly (which I don't think used to be possible), and it's a hopeless case for poor old YAPE::Regex::Explain.

I too am confused by why he demonstrated /@{[s".."."gr]}\K$"//; I don't see it mentioned among the submissions I viewed, and I don't see any mechanism for it actually doing what it's supposed to. It certainly doesn't match the code in the link provided.

Neigher \K nor /r existed in Perl 5.6.x, which is the latest version of the regexp syntax that YAPE::Regex::Explain supports. (This is documented, and would have been pretty easy to test.)

Yes, my bad.

I made up for it by providing a proper, detailed answer below... :)

It certainly doesn't match the code in the link provided.

Sorry. Link was wrong but code is OK.

Good link http://golf.shinh.org/reveal.rb?Bisection/tails+%28%3F%29_1377006539&pl

English is not my mother tongue.
Les tongues de ma mère sont "made in France".

PS: The s/@{[s".."."gr]}\K$"// solution does not work for me at all. Does it actually work for you?

Sure. This solution was accepted on anarchy golf site.

perl -e '$_="abcdef";s/@{[s".."."gr]}\K$"//;print"$_\n";' 
abc def
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