[R] help with regexp

[Gabor Grothendieck]

On Wed, Oct 5, 2011 at 7:56 AM, Jannis <bt_jannis at yahoo.de> wrote:
> Dear list memebers,
>
>
> I am stuck with using regular expressions.
>
>
> Imagine I have a vector of character strings like:
>
> test <- c('filename_1_def.pdf', 'filename_2_abc.pdf')
>
> How could I use regexpressions to extract only the 'def'/'abc' parts of these strings?
>
>
> Some try from my side yielded no results:
>
> testresults <- grep('(?<=filename_[[:digit:]]_).{1,3}(?=.pdf)', perl = TRUE, value = TRUE)
>
> Somehow I seem to miss some important concept here. Until now I always used nested sub expressions like:
>
> testresults <- sub('.pdf$', '', sub('^filename_[[:digit:]]_', '' , test))
>
>
> but this tends to become cumbersome and I was wondering whether there is a more elegant way to do this?
>

Here are a couple of solutions:

# remove everything up to _b as well as everything from . onwards
gsub(".*_|[.].*", "", test)

# extract everything that is not a _ provided it is immediately followed by .
library(gsubfn)
strapply(test, "([^_]+)[.]", simplify = TRUE)

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