using package tm to find phrases

I am using the package "tm" for text-mining of abstracts and would like to use
it to find instances of gene names that may contain white space. For instance
"gene regulatory protein 1". The default behavior of tm is to parse this into 4
separate words, but I would like to use the class constructor "dictionary" to
define phrases such as just mentioned.

Is this possible? If so, how?

Yes.

* In case you only need to find instances, you could use full text
  search on your corpus, e.g.

  R> tmIndex(yourCorpus, "gene regulatory protein 1")

  would return the indices of all documents in your corpus containing
  this phrase.

* If you need tokens (in a term-document matrix) of length 4, you could
  use a n-gram tokenizer (n = 4). See e.g.,
  http://tm.r-forge.r-project.org/faq.html#Bigrams. Then you can use
  the dictionary argument to store only your selection of gene
  names. I.e., something like

  R> yourTokenizer <- function(x) RWeka::NGramTokenizer(x, Weka_control(min = 4, max = 4))
  R> TermDocumentMatrix(crude, control = list(tokenize = yourTokenizer, dictionary = yourDictionary))

  where yourDictionary contains the gene names (a character vector
  suffices) to be included in the term-document matrix.

* If you want to extract arbitrary patterns of different length that
  could match some gene names (and build a dictionary from that), you
  need some custom functionality. Regular expressions might be a good
  starting point ...

Best regards, Ingo

Ingo Feinerer
Vienna University of Technology
http://www.dbai.tuwien.ac.at/staff/feinerer

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