Regex for ^ (the caret symbol)?
On 13-01-21 3:20 PM, Jeff Newmiller wrote:
Apparently Extended RegExp syntax eliminated the "^-is-an-ordinary-character-except-for-two-uses" meaning that I am familiar with from the Basic RegExp usage, since GNU grep with the -e option also refuses to match the carat unless it is escaped. The TRE library treats BRE as obsolete, so we only get ERE and Perl regexes in R. So I guess it isn't a bug, but is rather a "feature".
I re-read the ?regex help page, and I think it does actually say this, so we don't even have a documentation error as I thought before. When it is saying that ^ is a plain character except when it comes first, it is talking about first within a character class, e.g. [a^] meaning "a" or "^" as opposed to [^a] meaning "not a". Duncan Murdoch ---------------------------------------------------------------------------
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Duncan Murdoch <murdoch.duncan at gmail.com> wrote:
On 13-01-21 1:05 PM, Jeff Newmiller wrote:
So what is the special behavior of the ^ symbol when not at the
beginning of the string that occurs when it is not escaped? I think it retains its meaning as an assertion that it occurs at the beginning of the line, and so a pattern like "a^b" could never match anything. It's not very useful in this context, but I expect it's easier to implement in the case of complicated patterns, where some paths through the pattern put it at the beginning and others don't, e.g. (a|)^b has two possible patterns: a^b and ^b. Duncan Murdoch
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Sent from my phone. Please excuse my brevity. Duncan Murdoch <murdoch.duncan at gmail.com> wrote:
On 13-01-21 11:48 AM, Jeff Newmiller wrote:
I am not sure I understand what worked perfectly, since it is my
understanding that ^ is only special at the beginning of the regex
(to
anchor the pattern at the beginning of the target string) or as the first character of a character set (to indicate exclusion of the
listed
characters). In any other position the caret should behave like an ordinary character. That is, your original pattern should have
worked
as-is. This is supported by the help page documentation for regex in the paragraph below the definition of [:xdigit:]. I think this is a
bug
in R. It's a documentation error rather than a bug. The ^ character is special anywhere in the extended RE syntax defined by the TRE
library
or the Perl-compatible library that we use. This is inconsistent
with
the POSIX standard, which might be what you were thinking of. Duncan Murdoch
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Sent from my phone. Please excuse my brevity. mtb954 at gmail.com wrote:
Hi Tsjerk, many thanks...that worked perfectly! Mark Na On Mon, Jan 21, 2013 at 9:36 AM, Tsjerk Wassenaar
<tsjerkw at gmail.com>
wrote:
Oh, I'm jetlagged. ^ is a control character for 'start of
string'.
In
the
context of a character set it means negation: [^a-z]. Ciao, Tsjerk On Mon, Jan 21, 2013 at 4:33 PM, Tsjerk Wassenaar
<tsjerkw at gmail.com>wrote:
Hi Mark Na,
Try:
grepl("latitude\\^2",temp)
^ is a control character for negation, so you have to escape it.
Cheers,
Tsjerk
On Mon, Jan 21, 2013 at 4:26 PM, <mtb954 at gmail.com> wrote:
Hello R-helpers, I am trying to search for string that includes the caret
symbol,
using
the
following code:
grepl("latitude^2",temp)
And R doesn't like that. It gives me:
temp<-c("latitude^2","latitude and
latitude^2","longitude^2","longitude
and longitude^2")
temp
[1] "latitude^2" "latitude and latitude^2"
"longitude^2"
"longitude and longitude^2"
grepl("latitude^2",temp)
[1] FALSE FALSE FALSE FALSE I think this must a regex problem, but I can't find out to
specify
the
caret using regex.
I would appreciate any help you could provide.
Many thanks,
Mark Na
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______________________________________________ R-help at 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.
-- Tsjerk A. Wassenaar, Ph.D. post-doctoral researcher Biocomputing Group Department of Biological Sciences 2500 University Drive NW Calgary, AB T2N 1N4 Canada
-- Tsjerk A. Wassenaar, Ph.D. post-doctoral researcher Biocomputing Group Department of Biological Sciences 2500 University Drive NW Calgary, AB T2N 1N4 Canada
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______________________________________________ R-help at 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 at 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.