Re: [Qemu-devel] [PATCH] qmp-shell: fix nested json regression

[Kashyap Chamarthy <kchamart@redhat.com>]

On Wed, Feb 06, 2019 at 11:48:51AM +0100, Kashyap Chamarthy wrote:
> On Tue, Feb 05, 2019 at 08:44:12PM -0500, John Snow wrote:
> > On 2/5/19 8:49 AM, Marc-André Lureau wrote:

[...]

> > >              < command-name > [ arg-name1=arg1 ] ... [ arg-nameN=argN ]
> > >          """
> > > -        cmdargs = shlex.split(cmdline)
> > > +        cmdargs = re.findall(r'''(?:[^\s"']|"(?:\\.|[^"])*"|'(?:\\.|[^'])*')+''', cmdline)

Dan Berrangé explained on IRC this way:

    In plain english what that is saying is: give me all blocks of text
    which are:
    
      (a) set of chars not including " or '
      
      (b) a set of chars surrounded by ".." 
      
      (c) a set of chars surrounded by '...  

> > It might really be nice to have a comment briefly explaining the regex.
> > This is pretty close to symbol soup.
> 
> Yeah, a little comment explaining it would be nice.  

For my own education today I learned that ?: is called a "non-capturing
match".  For reference, quoting from the `perldoc perlretut`:

[quote]

  Non-capturing groupings
    A group that is required to bundle a set of alternatives may or may not
    be useful as a capturing group. If it isn't, it just creates a
    superfluous addition to the set of available capture group values,
    inside as well as outside the regexp. Non-capturing groupings, denoted
    by "(?:regexp)", still allow the regexp to be treated as a single unit,
    but don't establish a capturing group at the same time. Both capturing
    and non-capturing groupings are allowed to co-exist in the same regexp.
    Because there is no extraction, non-capturing groupings are faster than
    capturing groupings. Non-capturing groupings are also handy for choosing
    exactly which parts of a regexp are to be extracted to matching
    variables:

        # match a number, $1-$4 are set, but we only want $1
        /([+-]?\ *(\d+(\.\d*)?|\.\d+)([eE][+-]?\d+)?)/;

        # match a number faster , only $1 is set
        /([+-]?\ *(?:\d+(?:\.\d*)?|\.\d+)(?:[eE][+-]?\d+)?)/;

        # match a number, get $1 = whole number, $2 = exponent
        /([+-]?\ *(?:\d+(?:\.\d*)?|\.\d+)(?:[eE]([+-]?\d+))?)/;

    Non-capturing groupings are also useful for removing nuisance elements
    gathered from a split operation where parentheses are required for some
    reason:

        $x = '12aba34ba5';
        @num = split /(a|b)+/, $x;    # @num = ('12','a','34','a','5')
        @num = split /(?:a|b)+/, $x;  # @num = ('12','34','5')

[/quote]

[...]

-- 
/kashyap