use YAPE::Regex::Explain;
print YAPE::Regex::Explain->new(
qr/^>.*\n(^(?!>).*$)+/m
)->explain;
__END__
The regular expression:

(?m-isx:^>.*\n(^(?!>).*$)+)

matches as follows:
  
NODE                     EXPLANATION
----------------------------------------------------------------------
(?m-isx:                 group, but do not capture (with ^ and $
                         matching start and end of line) (case-
                         sensitive) (with . not matching \n)
                         (matching whitespace and # normally):
----------------------------------------------------------------------
  ^                        the beginning of a "line"
----------------------------------------------------------------------
  >                        '>'
----------------------------------------------------------------------
  .*                       any character except \n (0 or more times
                           (matching the most amount possible))
----------------------------------------------------------------------
  \n                       '\n' (newline)
----------------------------------------------------------------------
  (                        group and capture to \1 (1 or more times
                           (matching the most amount possible)):
----------------------------------------------------------------------
    ^                        the beginning of a "line"
----------------------------------------------------------------------
    (?!                      look ahead to see if there is not:
----------------------------------------------------------------------
      >                        '>'
----------------------------------------------------------------------
    )                        end of look-ahead
----------------------------------------------------------------------
    .*                       any character except \n (0 or more times
                             (matching the most amount possible))
----------------------------------------------------------------------
    $                        before an optional \n, and the end of a
                             "line"
----------------------------------------------------------------------
  )+                       end of \1 (NOTE: because you are using a
                           quantifier on this capture, only the LAST
                           repetition of the captured pattern will be
                           stored in \1)
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------