Dear all, It sounds as if regexp works according to the same rules as Perl, very nicely explained in: http://blob.perl.org/books/beginning-perl/3145_Chap05.pdf
Yet, I couldn't help but wonder if there are also differences in behaviour. I couldn't find any yet, but there must be some. Anybody care to elaborate on this? Cheers Joris On Wed, Jun 2, 2010 at 1:05 AM, Matt Shotwell <shotw...@musc.edu> wrote: > On Tue, 2010-06-01 at 16:43 -0400, Erik Iverson wrote: > > > > McGehee, Robert wrote: > > > R-help, > > > Sorry if this is more of a regex question than an R question. However, > > > help would be appreciated on my use of the regexpr function. > > > > > > In the first example below, I ask for all characters (a-z) in 'abc123'; > > > regexpr returns a 3-character match beginning at the first character. > > > > > >> regexpr("[[:alpha:]]*", "abc123") > > > [1] 1 > > > attr(,"match.length") > > > [1] 3 > > > > > > However, when the text is flipped regexpr, and I ask for a match of all > > > characters in '123abc', regexpr returns a zero-character match > beginning > > > at the first character. Can someone explain what a zero length match > > > means (i.e. why not return -1), and why the result isn't 4, > > > match.length=3? > > > > It means it matches 0 characters, which is fine since you use *, which > > means match 0 or more occurrences of the regex. It sounds like you want > > + instead of *. Also see gregexpr. > > Also, regular expressions try to match as early as possible. That's why > the match is at position one of length zero, and not at position four of > length three. > > Matt Shotwell > Graduate Student > Division of Biostatistics and Epidemiology > Medical University of South Carolina > > > > > > >> regexpr("[[:alpha:]]*", "123abc") > > > [1] 1 > > > attr(,"match.length") > > > [1] 0 > > > > > > > ______________________________________________ > > 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. > > ______________________________________________ > 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. > -- Joris Meys Statistical Consultant Ghent University Faculty of Bioscience Engineering Department of Applied mathematics, biometrics and process control Coupure Links 653 B-9000 Gent tel : +32 9 264 59 87 joris.m...@ugent.be ------------------------------- Disclaimer : http://helpdesk.ugent.be/e-maildisclaimer.php [[alternative HTML version deleted]] ______________________________________________ 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.