[R] gregexpr in R 2.3.0 != gregexpr in R 2.4.0

[Gabor Grothendieck]

You can get that by using zero width lookahead assertions. They must
match but are not consuming so the next match will not be forced
to start past them.  See ?regex and
   http://www.regular-expressions.info/lookaround.html
for more.

gregexpr(" [a-z](?= [a-z] )", " a b c d e f ", perl = TRUE)


On 10/6/06, Stefan Th. Gries <stgries_lists at arcor.de> wrote:
> Hi all
>
> I have a question regarding differences in the way gregpexr works in R 2.3.0 and R 2.4.0.
>
> In R 2.3.0, this is what happens:
>
> > gregexpr(" [a-z] [a-z] ", " a b c d e f ", perl=T)
> [[1]]
> [1] 1 3 5 7 9
> attr(,"match.length")
> [1] 5 5 5 5 5
>
>
> ... while in R 2.4.0, this is what happens:
>
> > gregexpr(" [a-z] [a-z] ", " a b c d e f ", perl=T)
> [[1]]
> [1] 1 7
> attr(,"match.length")
> [1] 5 5
>
>
>
> Looking at the archives, I came across these sites where the reverse issue has been discussed before:
>
> http://finzi.psych.upenn.edu/R/Rhelp02a/archive/75843.html
> http://finzi.psych.upenn.edu/R/Rhelp02a/archive/76815.html
> http://finzi.psych.upenn.edu/R/Rhelp02a/archive/75846.html
>
> >From there, it seems as if the first result has been considered undesirable (apparently because it differs from Perl's output if not also for other reasons) and R. Gentleman wrote that "[t]his has been reverted in R-devel, so you should get the old behavior in it." However,
>
> (i) I could not find any announcement of that change in the change log (the news file at <https://svn.r-project.org/R/trunk/NEWS> or at <http://cran.r-project.org/src/base/NEWS>) so I am still not sure whether this change of behavior is in fact due to changes by the R Development Core Team or not. So, first question: is this change intended or not? (My system has not changed otherwise.)
>
> (ii) Since for some applications of mine the first behavior above was exactly what I needed, I now have the same (second) question as Thomas Girke before: is there a way to get the first of the two results now in R 2.4.0 (on a Windows XP machine)?
>
> Thanks a lot,
> STG
>
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