Re: [R] Regular expressions: bug or misunderstanding?

From: Ted Harding <Ted.Harding_at_manchester.ac.uk>
Date: Sun, 06 Jul 2008 22:37:13 +0100 (BST)


On 06-Jul-08 21:17:04, Duncan Murdoch wrote:
> I'm trying to write a gsub() call that takes a string and escapes all
> the unescaped quote marks in it. So the string
>
> \"
>
> would be left unchanged, but
>
> \\"
>
> would be changed to
>
> \\\"
>
> because the double backslash doesn't act as an escape for the quote,
> the first just escapes the second. I have the usual problems of
> writing regular expressions involving backslashes which make
> everything I write completely unreadable, so I'm going to change
> the problem for this post: I will define E to be the escape
> character, and q to be the quote; the gsub() call would leave
>
> Eq
>
> unchanged, but would change
>
> EEq
>
> to EEEq, etc.
>
> The expression I have come up with after this change is
>
> gsub( "((^|[^E])(EE)*)q", "\\1Eq", x)
>
> i.e. "(start of line, or non-escape, followed by an even number of
> escapes), all of which we call expression 1, followed by a quote,
> is replaced by expression 1 followed by an escape and a quote".
>
> This works sometimes, but not always:
>
> > gsub( "((^|[^E])(EE)*)q", "\\1Eq", "Eq")
> [1] "Eq"
> > gsub( "((^|[^E])(EE)*)q", "\\1Eq", "EEq")
> [1] "EEEq"
> > gsub( "((^|[^E])(EE)*)q", "\\1Eq", "qaq")
> [1] "EqaEq"
> > gsub( "((^|[^E])(EE)*)q", "\\1Eq", "qq")
> [1] "qEq"
>
> Notice that in the final example, the first quote doesn't get escaped.
> Why not????

I think (without having done the "experimental diagnostics") that it's because in "qq" the first q mtaches (^|[^E]) because it matches [^E] (i.e. is a "non-escape"); since it is followed by q, it is the second q which gets the escape. Possibly you need to include "^q" as an additional alternative match at the start of the line.

Ted.



E-Mail: (Ted Harding) <Ted.Harding_at_manchester.ac.uk> Fax-to-email: +44 (0)870 094 0861
Date: 06-Jul-08                                       Time: 22:37:10
------------------------------ XFMail ------------------------------

______________________________________________
R-help_at_r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code. Received on Sun 06 Jul 2008 - 21:41:48 GMT

Archive maintained by Robert King, hosted by the discipline of statistics at the University of Newcastle, Australia.
Archive generated by hypermail 2.2.0, at Mon 07 Jul 2008 - 00:31:42 GMT.

Mailing list information is available at https://stat.ethz.ch/mailman/listinfo/r-help. Please read the posting guide before posting to the list.

list of date sections of archive