import re

strings = ["Alpha_beta_Gamma", "Alpha_Beta_Gamma"]
pattern = r'^[A-Z][a-z]*(?:_[A-Z][a-z]*)*$'

for s in strings:
    if re.match(pattern, s):
        print s + " conforms"
    else:
        print s + " doesn't conform"

Alpha_beta_Gamma doesn't conform
Alpha_Beta_Gamma conforms