Seriously, parsing something that has balanced anythings with regular expressions is notoriously hard to get right, especially if you want to be able to report back exactly where your user went wrong. They're more of a yes-or-no kind of deal.

If you want to parse a command line, it's better to use an actual parsing routine. The one in tilly's Text::xSV should be very close to what you're looking for. Actually, you could probably use Text::xSV with some very slight modifications. Well, one modification - you need to be able to specify the SEP as a regex of \s+.

That's probably the best answer, IMHO. Here's a more hackish approach:

chomp( @a = qx( for x in $_; do echo \$x; done ) );

There's already a tool that knows how to parse command lines! :-)

Update: fixed bugs in code, which was untested. Thanks, tlm!

And in fact, these tools are bundled with Perl. Take a look at geopt/Getopt::Std, although I like Getopt::Long much better.

Another option is the core module Text::ParseWords, which includes a shellwords function that likely does just the right thing.

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]