[R] Named backreferences in replacement patterns

[Stefan Th. Gries]

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