import re
pat = r"\b([^\W\d_])([^\W\d_]*)([^\W\d_])\b"
s = "Testers"
print(re.sub(pat, (lambda m: "{0}{1}{2}".format(m.group(1), len(''.join(set(m.group(2)))), m.group(3))), s))
aW1wb3J0IHJlCnBhdCA9IHIiXGIoW15cV1xkX10pKFteXFdcZF9dKikoW15cV1xkX10pXGIiCnMgPSAiVGVzdGVycyIKcHJpbnQocmUuc3ViKHBhdCwgKGxhbWJkYSBtOiAiezB9ezF9ezJ9Ii5mb3JtYXQobS5ncm91cCgxKSwgbGVuKCcnLmpvaW4oc2V0KG0uZ3JvdXAoMikpKSksIG0uZ3JvdXAoMykpKSwgcykp