[R] Named backreferences in replacement patterns

[Gabor Grothendieck]

Sorry. An extra line got in there.  It should have been:

library(gsubfn)
gsubfn("(?<month>\\d+)/(?<day>\\d+)/",
  month + day ~ sprintf("%s/%s/", day, month),
  backref = -2, British.dates, perl = TRUE)



On 3/8/07, Gabor Grothendieck <ggrothendieck at gmail.com> wrote:
> This does not really answer the specific question you posted but
> gsubfn can do it without using \\1 and \\2 like this (you probably
> realized that already but I thought I would point it out just in  case):
>
> library(gsubfn)
> gsubfn("(?<month>\\d+)/(?<day>\\d+)/",
>   month + day ~ sprintf("%s/%s/", day, month),
>   British.dates,
>   backref = -2, British.dates, perl = TRUE)
>
> On 3/7/07, Stefan Th. Gries <stgries_lists at arcor.de> wrote:
> > Hi
> >
> > I have a problem with substitutions involving named backreferences. I
> > have a vector American.dates:
> >
> > > American.dates
> > [1] "5/15/1976" "2.15.1970" "1.9.2006"
> >
> > which I want to change into British.dates:
> >
> > > British.dates
> > [1] "15/5/1976" "15/2/1970" "9/1/2006"
> >
> > I know I can do it like this:
> >
> > British.dates<-sub("(\\d{1,2})\\D(\\d{1,2})\\D", "\\2/\\1/",
> > American.dates, perl=T)
> >
> > But I want to use named backreferences. I was trying something like
> > this
> >
> > British.dates<-sub("(?P<MONTH>\\d{1,2})\\D(?P<DAY>\\d{1,2})\\D",
> > "...", American.dates, perl=T)
> >
> > but the problem is the ... I know I could use the named backreferences
> > *in the search pattern* with (?P=MONTH), but how do I use them *in the
> > replacement pattern"? I didn't get the Python solution to work:
> >
> > > (British.dates<-sub("(?P<MONTH>\\d{1,2})\\D(?P<DAY>\\d{1,2})\\D", "'\g<MONTH>\g/\\1/", American.dates, perl=T))
> > [1] "'g<MONTH>g/5/1976" "'g<MONTH>g/2/1970" "'g<MONTH>g/1/2006"
> >
> > > (British.dates<-sub("(?P<MONTH>\\d{1,2})\\D(?P<DAY>\\d{1,2})\\D", "'\\g<MONTH>\\g/\\1/", American.dates, perl=T))
> > [1] "'g<MONTH>g/5/1976" "'g<MONTH>g/2/1970" "'g<MONTH>g/1/2006"
> >
> > I know I can use the numbers again, but then what would be the point
> > of having used names in the first place ...
> >
> > Any ideas? Thx a bunch,
> > STG
> >
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