import re
from collections import defaultdict
def group_by_first_n_letter(s, n=3):
words = re.findall(r"[a-zA-Z]+", s.lower())
groups = defaultdict(list)
for w in words:
if len(w) >= n:
groups[w[:n]].append(w)
return dict(groups)
s = "The lowly inhabitants of the lowland were surprised to see the lower branches of the trees."
groups = group_by_first_n_letter(s, 3)
for prefix, words in groups.items():
print(prefix, ":", words)
print()
for prefix, words in groups.items():
print(f"{prefix}: {', '.join(words)}")
'''
run:
the : ['the', 'the', 'the', 'the']
low : ['lowly', 'lowland', 'lower']
inh : ['inhabitants']
wer : ['were']
sur : ['surprised']
see : ['see']
bra : ['branches']
tre : ['trees']
the: the, the, the, the
low: lowly, lowland, lower
inh: inhabitants
wer: were
sur: surprised
see: see
bra: branches
tre: trees
'''
