Thanks, Gabor, for the initiation to perly regexps! I've only been used to extended ones till now. A pity, perhaps, that "perl=TRUE" is not an option for the likes of browseEnv(), help.search(), list.files() and ls() (which take extended regexps), but one can always assign the output and then grep(...,perl=TRUE) on that, as you illustrate.
It would seem that these "look-ahead" features cold allow quite complex logical conditions to be built up (though with increasing unreadability and head-scratching)! Best wishes, Ted. On 12-Jun-10 14:10:13, Gabor Grothendieck wrote: > On Sat, Jun 12, 2010 at 5:38 AM, Ted Harding > <ted.hard...@manchester.ac.uk> wrote: >> Greetings, >> The following question has come up in an off-list discussion. >> Is it possible to construct a regular expression 'rex' out of >> two given regular expressions 'rex1' and 'rex2', such that a >> character string X matches 'rex' if and only if X matches 'rex1' >> AND X does not match 'rex2'? >> >> The desired end result can be achieved by logically combining >> the results of a grep using 'rex1' with the results of a grep >> on 'rex2', illustrated by the following example: >> >> ## Given character vector X (below), and two regular exdpressions >> ## rex1="abc", rex2="ijk", to return the elements of X which match >> ## rex1 AND do not match rex1: >> X <- c( >> _"abcdefg", _ _ _ # Yes >> _"abchijk", _ _ _ # No >> _"mnopqrs", _ _ _ # No >> _"ijkpqrs", _ _ _ # No >> _"abcpqrs" ) _ _ _# Yes >> rex1 <- "abc" >> rex2 <- "ijk" >> ix1<- grep(rex1,X) >> ix2<- grep(rex2,X) >> X[ix1[!(ix1 %in% ix2)]] >> ## [1] "abcdefg" "abcpqrs" >> >> Question: is there a way to construct 'rex' from 'rex1' and 'rex2' >> such that >> >> _X[grep(rex,X)] >> >> would given the same result? > > Try this: > > rex <- "^(?!(.*ijk)).*abc" > grep(rex, X, perl = TRUE) > > Also note that X[grep(rex, X, perl = TRUE)] can be written: > > grep(rex, X, perl = TRUE, value = TRUE) > > See ?regex for more info. Further regular expression links can be > found in the External Links box on the gsubfn home page at > http://gsubfn.googlecode.com > > ______________________________________________ > R-help@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. -------------------------------------------------------------------- E-Mail: (Ted Harding) <ted.hard...@manchester.ac.uk> Fax-to-email: +44 (0)870 094 0861 Date: 12-Jun-10 Time: 16:46:56 ------------------------------ XFMail ------------------------------ ______________________________________________ R-help@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.
