On Sat, Nov 8, 2008 at 9:41 AM, Duncan Murdoch <[EMAIL PROTECTED]> wrote: > On 08/11/2008 7:20 AM, John Wiedenhoeft wrote: >> >> Hi there, >> >> I rejoiced when I realized that you can use Perl regex from within R. >> However, as the FAQ states "Some functions, particularly those involving >> regular expression matching, themselves use metacharacters, which may need >> to be escaped by the backslash mechanism. In those cases you may need a >> quadruple backslash to represent a single literal one. " >> >> I was wondering if that is really necessary for perl=TRUE? wouldn't it be >> possible to parse a string differently in a regex context, e.g. >> automatically insert \\ for each \ , such that you can use the perl syntax >> directly? For example, if you want to input a newline as a character, you >> would use \n anyway. At the moment one says \\n to make it clear to R that >> you mean \n to make clear that you mean newline... this is pretty annoying. >> How likely is it that you want to pass a real newline character to PCRE >> directly? > > No, that's not possible. At the level where the parsing takes place R has > no idea of its eventual use, so it can't tell that some strings are going to > be interpreted as Perl, and others not. > > As Gabor mentioned, there have been various discussions of adding a new > syntax for strings that are parsed literally, without processing any > escapes, but no consensus on the right syntax to use. > > There are currently some fragile tricks that let you avoid escapes, e.g. > using scan() to read a line: > >> re <- scan(what="", n=1) > 1: [^\\] > Read 1 item >> re > [1] "[^\\\\]" > > (I call this fragile because it works in scripts processed at console level, > but not if you type the same thing into a function.) > > So I agree, it would be nice to have new syntax to allow this. Last time > this came up, I argued for something like \verb in LaTeX where the delimiter > could be specified differently in each use. Duncan TL suggested triple > quotes, as in Python. I think now that triple quotes would be be better > than the particular form I suggested.
Ruby's quoting method looks quite flexible: http://en.wikibooks.org/wiki/Ruby_Programming/Alternate_quotes ______________________________________________ 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.
