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 (casesensitive)
(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

