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
