help with regexp

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

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?

Von: Gabor Grothendieck <ggrothendieck at gmail.com>
Betreff: Re: [R] help with regexp
An: "Jannis" <bt_jannis at yahoo.de>
CC: r-help at stat.math.ethz.ch
Datum: Mittwoch, 5. Oktober, 2011 15:13 Uhr
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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