Natural language processing

What can be done with it? Simple programs? Python constructs? What tools and techniques does Python provide for such work? Interesting challenges of natural language processing? Pitfalls?

My getting started with Python is here. There are several open source Natural Language Processing (NLP) libraries. The most used are:

  • Natural language toolkit (NLTK)
  • Apache OpenNLP
  • Stanford NLP suite
  • Gate NLP library

These are things one can do with NLTK: