import re  
def text_match(text):  
        patterns = '^[a-z]+_[a-z]+$'  
        if re.search(patterns,  text):  
                return 'Found a match!'  
        else:  
                return('Not matched!')  
  
print(text_match("aab_cbbbc"))  
print(text_match("aab_Abbbc"))  
print(text_match("Aaab_abbbc"))