Python script to print a counted list of all digraphs in a set of files

digraphs.py

#!/usr/bin/env python

import fileinput, re

counts = {}

for l in fileinput.input():
    l = l.lower()
    for m in re.finditer(r'[a-z](?=[a-z])', l):
        graph = l[m.start():m.end()+1]
        counts[graph] = counts.get(graph, 0) + 1

# Output in frequency order, highest first
for k, v in sorted(counts.items(), key=lambda (k, v): v, reverse=True):
    print k, v

@neilmayhew doesn't the [a-z] portion exclude punctuation marks and capitals?