[R] Regular expressions: retrieving matches depending on intervening strings [Follow-up]

[Stefan Th. Gries]

Dear all

This is a follow-up to an earlier posting today regarding a regular expression question. In the meantime, this is the best approximation I could come up with and should give you a better idea what I am talking about.

a<-c("<w AT0>a <w NN1>blockage <w CJC>and <w DT0>that<c PUN>.",
     "<w AT0>a <w NN1>blockage <w CJC>and <ptr target=KB2LC003><w DT0>that<c PUN>.",
     "<w AT0>a <w NN1>blockage <w CJC>and<c PUN>, <w DT0>that<c PUN>.",
     "<w AT0>a <w NN1>blockage <w CJC>and <w AJ0>hungry <w DT0>that<c PUN>.")
matches<-gregexpr("<w CJC>[^<]+(?:<[^wc].*?>.*?)*<w DT0>that", a, perl=TRUE)
starts<-unlist(matches)
lengths<-unlist(sapply(matches, attributes))
stops<-starts+lengths-1
substr(a, starts, stops)

What is still missing is that the disallowed string is not just "<[wc]" but "<[wc] " and I don't know how to do that. Any ideas (preferably with lookarounds)?
Thanks a bunch,
STG
--
Stefan Th. Gries
-----------------------------------------------
University of California, Santa Barbara
http://www.linguistics.ucsb.edu/faculty/stgries
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ORIGINAL MESSAGE
> Dear all
>
> I again have a regular expression question. I have this character vector a:
>
> a<-c("<w AT0>a <w NN1>blockage <w CJC>and <w DT0>that<c PUN>.",
> "<w AT0>a <w NN1>blockage <w CJC>and <ptr target=KB2LC003><w DT0>that<c PUN>.",
> "<w AT0>a <w NN1>blockage <w CJC>and<c PUN>, <w DT0>that<c PUN>.",
> "<w AT0>a <w NN1>blockage <w CJC>and <w AJ0>hungry <w DT0>that<c PUN>.")
>
> I would like to retrieve those elements of a in which "<w CJC>" and "<w DT0>" are
>
> - directly adjacent, as in a[1] or
> - not interrupted by "<[wc] ", as in a[2]
>
> And, of these elements I would like to consume all characters from the "<" in "<w CJC" to the last character after "<w DT0>" that is not a "<". For example, if I was only searching a[1], I would like something like this:
>
> matches<-gregexpr("<w CJC>[^<]+?<w DT0>[^<]+", a[1], perl=TRUE)
> substr(a[1], unlist(matches), unlist(matches)+unlist(attributes(matches[[1]], "match.length"))-1)
>
> I have been fiddling around with negative lookahead but I really can't get my head around this. Any pointers would be greatly appreciated. Thanks a lot,
> STG