import re
pattern = r"_?(\d+)|_"
i = "fields_5_type_fields_1_name"
res = re.sub(pattern, lambda x: f"[{x.group(1)}]" if x.group(1) else ".", i)
print(res)
aW1wb3J0IHJlCgpwYXR0ZXJuID0gciJfPyhcZCspfF8iCmkgPSAiZmllbGRzXzVfdHlwZV9maWVsZHNfMV9uYW1lIgpyZXMgPSByZS5zdWIocGF0dGVybiwgbGFtYmRhIHg6IGYiW3t4Lmdyb3VwKDEpfV0iIGlmIHguZ3JvdXAoMSkgZWxzZSAiLiIsIGkpCnByaW50KHJlcyk=