The regular expressions in compareVersion()

On 24/04/2014, 10:27 PM, Simon Urbanek wrote:

FWIW the link has a long thread that is 90% irrelevant - AFAICS the relevant part is

From: Yihui Xie-2
Sep 02, 2013; 4:11pm
Re: Sweave: printing an underscore in the output from an R command
[...]
Now you are good at the regular expression level, but Sweave comes and
bites you, and that is due to this bug in the regular expression in
Sweave Noweb syntax:

SweaveSyntaxNoweb$docexpr

[1] "\\\\Sexpr\\{([^\\}]*)\\}"

It should have been "\\\\Sexpr\\{([^}]*)\\}", i.e. } does not need to
be escaped inside [], and \\ will be interpreted literally inside [].
In your case, Sweave sees \ in \Sexpr{}, and the regular expression
stops matching there, and is unable to see } after \, so it believes
there is no inline R expressions in your document.

Thanks.  I've put in a bug report on this one now, so it shouldn't get missed again.  If nobody else gets to it first I'll deal with it.

I don't see any value in fixing the compareVersion example, but if someone submits a bug report about it, someone else might fix it.

Duncan Murdoch

On Apr 24, 2014, at 10:15 PM, Yihui Xie <xie at yihui.name> wrote:

You are right that this is unlikely to cause problems, because users
are unlikely to put backslashes in version numbers. Henrik has pointed
out the problem. It is not about "making the source code a little
cleaner", but "making it correct". Either someone in R core corrects
the wrong regular expressions in a few seconds (unless you think \ can
be a legal character in a version number), or I just give up the
report. It seems the latter is easier. It is not worth additional
Q&A's back and forth.

Regarding the regular expression problem for \Sexpr{} in Sweave,
please see here for a record:
http://r.789695.n4.nabble.com/Sweave-printing-an-underscore-in-the-output-from-an-R-command-td4675177.html
As I said, it is a similar problem: someone tried to escape a
character that did not need to be escaped in [].

Regards,
Yihui
--
Yihui Xie <xieyihui at gmail.com>
Web: http://yihui.name


On Thu, Apr 24, 2014 at 6:20 PM, Duncan Murdoch
<murdoch.duncan at gmail.com> wrote:

On 24/04/2014, 5:26 PM, Henrik Bengtsson wrote:

On Thu, Apr 24, 2014 at 1:42 PM, Duncan Murdoch
<murdoch.duncan at gmail.com> wrote:

On 24/04/2014, 1:11 PM, Yihui Xie wrote:

Hi,

I guess the backslash should not be used as the separator for
strsplit() in compareVersion(), because the period in [.] is no longer
a metacharacter (no need to "escape" it using a backslash):


https://github.com/wch/r-source/blob/trunk/src/library/utils/R/packages.R#L866-L867

compareVersion

function (a, b)
{
....
     a <- as.integer(strsplit(a, "[\\.-]")[[1L]])
     b <- as.integer(strsplit(b, "[\\.-]")[[1L]])
....
<environment: namespace:utils>

Could you post an example where this causes trouble, or are you just
suggesting this as a way to make the source a little cleaner?

Maybe it's already clear, but [\\.] is the set for the two symbols '\'
and '.', not '.' alone.  For example, I would expect an error below:

compareVersion("3.14-59.26", "3.14-59\\26")

[1] 0

How does that cause problems?

Duncan Murdoch

/Henrik

A similar regular expression problem also exists in the Sweave syntax
(for \Sexpr{}), and I have reported it once. It was fixed but the fix
was immediately reverted for some reason:


https://github.com/wch/r-source/commit/52b0a46e15136a7f9e4777e9960fdda6d84880c0

A link to your report would be more useful, if it included an example
where
the bad regexp causes trouble.

Duncan Murdoch