[R] help with regexp

Eik Vettorazzi E.Vettorazzi at uke.de
Wed Oct 5 15:11:30 CEST 2011


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.

Am 05.10.2011 13:56, schrieb 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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Eik Vettorazzi
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Universitätsklinikum Hamburg-Eppendorf

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