aolle

code1.py

import re
s = 'aaaaabbbbccccccaaaaaaa'
d = tuple((c[0], len(c)) for c in [m.group(0) for m in re.finditer(r"(\w)\1*", s)])
l = [(str(t[1]),t[0]) for t in d]
''.join(str(j) for i in l for j in i)


Coarse grained benchmark time: 0.028s

aolle

import re

s = 'aaabbbaaa'
d = tuple((c[0], len(c)) for c in [m.group(0) for m in re.finditer(r"(\w)\1*", s)])
l = [(str(t[1]),t[0]) for t in d]
''.join(str(j) for i in l for j in i)