Byte Pair Encoding example (Source: Sennrich et al. - https://arxiv.org/abs/1508.07909)

BPE

import re, collections

def get_stats(vocab):
   pairs = collections.defaultdict(int)
   for word, freq in vocab.items():
       symbols = word.split()
       for i in range(len(symbols)-1):
           pairs[symbols[i],symbols[i+1]] += freq
   return pairs

def merge_vocab(pair, v_in):
   v_out = {}
   bigram = re.escape(' '.join(pair))
   p = re.compile(r'(?<!\S)' + bigram + r'(?!\S)')
   for word in v_in:
       w_out = p.sub(''.join(pair), word)
       v_out[w_out] = v_in[word]
   return v_out

vocab = {'l o w </w>' : 5, 'l o w e r </w>' : 2, 'n e w e s t </w>':6, 'w i d e s t </w>':3}
num_merges = 6

for i in range(num_merges):
   pairs = get_stats(vocab)
   best = max(pairs, key=pairs.get)
   vocab = merge_vocab(best, vocab)
   print(best)