replace operator to replace n't by " not" (space not)

I am doing Sentiment Analysis of twitter data. now in the data processing part I wanted to remove the negation words like couldn't, wouldn't, aren't, etc. except the word won't. So I thought of using the replace operator to replace n't by  " not" (space not). This should find n't in words like couldn't and as soon as it finds it, it replaces by space not making couldn't look like could not. But I'm not yielding the desired results. Can you help me with this, as to what will be the correct regular expression for replacing n't with space not?