使用python在文件中计数bigrams(对两个词)

I want to count the number of occurrences of all bigrams (pair of adjacent words) in a file using python. Here, I am dealing with very large files, so I am looking for an efficient way. I tried using count method with regex "\w+\s\w+" on file contents, but it did not prove to be efficient.I want to count the number of occurrences of al