use YAPE::Regex::Explain;
print YAPE::Regex::Explain->new(
qr/([^\r\n]+)$/s
)->explain;
__END__
The regular expression:

(?s-imx:([^\r\n]+)$)

matches as follows:

NODE                     EXPLANATION
----------------------------------------------------------------------
(?s-imx:                 group, but do not capture (with . matching
                         \n) (case-sensitive) (with ^ and $ matching
                         normally) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  (                        group and capture to \1:
----------------------------------------------------------------------
    [^\r\n]+                 any character except: '\r' (carriage
                             return), '\n' (newline) (1 or more times
                             (matching the most amount possible))
----------------------------------------------------------------------
  )                        end of \1
----------------------------------------------------------------------
  $                        before an optional \n, and the end of the
                           string
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------