Just to break it up a little, YAPE::Regex::Explain produces this:

The regular expression:

(?-imsx:(?:([^"\s]+)|\s+|("[^"\\]*(?:\\.[^"\\]*)*"))+)

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, but do not capture (1 or more times
                           (matching the most amount possible)):
----------------------------------------------------------------------
    (                        group and capture to \1:
----------------------------------------------------------------------
      [^"\s]+                  any character except: '"', whitespace
                               (\n, \r, \t, \f, and " ") (1 or more
                               times (matching the most amount
                               possible))
----------------------------------------------------------------------
    )                        end of \1
----------------------------------------------------------------------
   |                        OR
----------------------------------------------------------------------
    \s+                      whitespace (\n, \r, \t, \f, and " ") (1
                             or more times (matching the most amount
                             possible))
----------------------------------------------------------------------
   |                        OR
----------------------------------------------------------------------
    (                        group and capture to \2:
----------------------------------------------------------------------
      "                        '"'
----------------------------------------------------------------------
      [^"\\]*                  any character except: '"', '\\' (0 or
                               more times (matching the most amount
                               possible))
----------------------------------------------------------------------
      (?:                      group, but do not capture (0 or more
                               times (matching the most amount
                               possible)):
----------------------------------------------------------------------
        \\                       '\'
----------------------------------------------------------------------
        .                        any character except \n
----------------------------------------------------------------------
        [^"\\]*                  any character except: '"', '\\' (0
                                 or more times (matching the most
                                 amount possible))
----------------------------------------------------------------------
      )*                       end of grouping
----------------------------------------------------------------------
      "                        '"'
----------------------------------------------------------------------
    )                        end of \2
----------------------------------------------------------------------
  )+                       end of grouping
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------
[download]

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