[R] Question on Stopword Removal from a Cyrillic (Bulgarian)Text

[Ventseslav Kozarev]

I just wanted to confirm that Milan's suggestion about adding (*UCP) like in 
the example below:

gsub(sprintf("(*UCP)\\b(%s)\\b", "който"), "", "който", perl=TRUE)

solved all problems (under openSuse Linux 12.3 64-bit, R 2.15.2). I reencoded 
input files and stop word list in UTF-8, and now stop words are properly 
removed using the suggested syntax:

sme.corpus<-tm_map(sme.corpus,removeWords.PlainTextDocument,stoplist)

where:
removeWords.PlainTextDocument <- function (x, words)
  gsub(sprintf("(*UCP)\\b(%s)\\b", paste(words, collapse = "|")), "", x, 
perl=TRUE)

and stoplist is a character vector of stop words.

The wordcloud function now also accept the preprocessed corpus without 
warnings or errors. Now, if only I could do stemming in Bulgarian, that would 
have been priceless!

Thanks again, this has been tremendous help indeed!
Vince


On Wednesday 10 April 2013 20:43:27 Milan Bouchet-Valat wrote:
> Le mercredi 10 avril 2013 à 13:17 +0200, Ingo Feinerer a écrit :
> > On Wed, Apr 10, 2013 at 10:29:27AM +0200, Milan Bouchet-Valat wrote:
> > > Thanks for the reproducible example. Indeed, it does not work here
> > > either (Linux with UTF-8 locale). The problem seems to be in the call to
> > > gsub() in removeWords: the pattern "\\b" does not match anything when
> > > perl=TRUE. With perl=FALSE, it works.
> > 
> > The \b versus perl versus UTF-8 issue seems to be known, and it is
> > advised to use perl = TRUE with \b. See e.g. the warning in the gsub
> > help page (?gsub):
> > 
> > ---8<---------------------------------------------------------------------
> > ----- Warning:
> > 
> > POSIX 1003.2 mode of ‘gsub’ and ‘gregexpr’ does not work correctly with
> > repeated word-boundaries (e.g. ‘pattern = "\b"’).  Use ‘perl = TRUE’ for
> > such matches (but that may not work as expected with non-ASCII inputs,
> > as the meaning of ‘word’ is system-dependent).
> > ---8<---------------------------------------------------------------------
> > -----
> Thanks for the pointer. Indeed, this allowed me to discover the
> existence of the PCRE_UCP (Unicode Character Properties) flag, which
> changes matching behavior so that Unicode alphanumerics are not
> considered as word boundaries.
> 
> This flag should probably be used by R when calling pcre_compile() in
> gsub() and friends. At the moment, R's behavior is inconsistent across
> platforms:
> - on Fedora 18, R 2.15.3 :
> gsub("\\bt\\b", "", "télégramme", perl=TRUE)
> [1] "élégramme"
> 
> - on Windows 2008, R 2.15.1 and 3.0.0 :
> gsub("\\bt\\b", "", "télégramme", perl=TRUE)
> [1] "télégramme"
> 
> 
> Luckily, the bug can be fixed at tm's level by adding (*UCP) at the
> 
> beginning of the pattern. This works for our examples :
> > gsub(sprintf("\\b(%s)\\b", "който"), "", "който", perl=TRUE)
> 
> [1] "който"
> 
> > gsub(sprintf("(*UCP)\\b(%s)\\b", "който"), "", "който", perl=TRUE)
> 
> [1] ""
> 
> gsub("\\bt\\b", "", "télégramme", perl=TRUE)
> [1] "élégramme"
> gsub("(*UCP)\\bt\\b", "", "télégramme", perl=TRUE)
> [1] "télégramme"
> 
> 
> Regards