import re
pattern = r'_?[A-Z]{2,}_?|([A-Z](?:_[A-Z](?![A-Z]))+)'
s = ("A_B_CDEF_GHI\n"
"A_B_C_DEF_G_H_I\n"
"ABC_D_E_F_GHI\n"
"ABCDEFG_H_I\n"
"A_B_C")
res = re.sub(pattern, lambda x: x.group(1).replace("_", "") if x.group(1) else x.group(), s)
print(res)
aW1wb3J0IHJlCnBhdHRlcm4gPSByJ18/W0EtWl17Mix9Xz98KFtBLVpdKD86X1tBLVpdKD8hW0EtWl0pKSspJwpzID0gKCJBX0JfQ0RFRl9HSElcbiIKICAgICAgICAgICAgIkFfQl9DX0RFRl9HX0hfSVxuIgogICAgICAgICAgICAiQUJDX0RfRV9GX0dISVxuIgogICAgICAgICAgICAiQUJDREVGR19IX0lcbiIKICAgICAgICAgICAgIkFfQl9DIikKCnJlcyA9IHJlLnN1YihwYXR0ZXJuLCBsYW1iZGEgeDogeC5ncm91cCgxKS5yZXBsYWNlKCJfIiwgIiIpIGlmIHguZ3JvdXAoMSkgZWxzZSB4Lmdyb3VwKCksIHMpCnByaW50KHJlcyk=