[R] Parsing regular expressions differently - feature request
ggrothendieck at gmail.com
Sat Nov 8 17:03:48 CET 2008
On Sat, Nov 8, 2008 at 9:41 AM, Duncan Murdoch <murdoch at stats.uwo.ca> 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
> 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
>  "[^\\\\]"
> (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:
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