[R] help with regexp

[Jannis]

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?

Thanks for any help

Jannis

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[Albert-Jan Roskam]

Hello!
 
library(gsubfn)

test <- c('filename_1_def.pdf', 'filename_2_abc.pdf')

gsubfn("(.+_)([a-z]+)(\\.pdf)", "\\2", test)

Cheers!!
Albert-Jan

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>________________________________
>From: Jannis <bt_j...@yahoo.de>
>To: r-h...@stat.math.ethz.ch
>Sent: Wednesday, October 5, 2011 1:56 PM
>Subject: [R] help with regexp

[[alternative HTML version deleted]]

[Eik Vettorazzi]

Hi Jannis,
just use the backreferences in gsub, see ?gsub, -> replacement

test <- c('filename_1_def.pdf', 'filename_2_abc.pdf')

gsub(".*_([A-z]+)\\.pdf", "\\1", test)

hth.

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[Gabor Grothendieck]

On Wed, Oct 5, 2011 at 7:56 AM, Jannis <bt_j...@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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email: ggrothendieck at gmail.com

[Jannis]

Thanks to all who replied! With all these possible solutions it will be hard to find the best one :-).

--- Gabor Grothendieck <ggroth...@gmail.com> schrieb am Mi, 5.10.2011: