> On Aug 23, 2017, at 2:29 AM, Rolf Turner <r.tur...@auckland.ac.nz> wrote:
>
>
> On 23/08/17 18:33, Stefan Evert wrote:
>
>>> On 23 Aug 2017, at 07:45, Rolf Turner <r.tur...@auckland.ac.nz> wrote:
>>>
>>> My reading of ?regex led me to believe that
>>>
>>> gsub("[:alpha:]","",x)
>>>
>>> should give the result that I want.
>> That's looking for any of the characters a, l, p, h, : .
>
> OK. I see that now. I don't think that it's really stated anywhere that to
> search for (and possibly change) any one of a string of characters you
> enclose that string of characters in brackets [ ].
That's explained on the ?regex page in the section on character classes. The
source of confusion for you is that within regex character classes there is
also a set of reserved constructions that all start and end with "[:" and ":]".
It's a bit like needed to double or triple escape characters in regex. a
leading "|" changes the parser settings (or "expectations" if one wants to
anthropomorphize the process.
>
> The first example from ?grep makes this "clear" (for some value of the word
> "clear") once you understand what this example is on about.
>
> So it's "obvious" once you've been shown, and totally opaque until then.
Sometimes we all stumble over syntactic "special" detours. If you wanted to add
a warning to the current ?regex tex, you could submit a diff for the base
package, perhaps with something like:
"Certain named classes of characters are predefined. Their interpretation
depends on the locale (see locales); the interpretation below is that of the
POSIX locale."
Replaced with:
"Certain named classes of characters are predefined. Their interpretation
depends on the locale (see locales); the interpretation below is that of the
POSIX locale. Their names do include the "[:" and ":]" characters."
>
>> What you meant to say was
>> gsub("[[:alpha:]]","",x)
>> i.e. the character class [:alpha:] within a character set.
>
> Yup. Got it. Thanks very much.
>
> cheers,
>
> Rolf
>
> --
> Technical Editor ANZJS
> Department of Statistics
> University of Auckland
> Phone: +64-9-373-7599 ext. 88276
>
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David Winsemius
Alameda, CA, USA
'Any technology distinguishable from magic is insufficiently advanced.'
-Gehm's Corollary to Clarke's Third Law
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