Find sequences of one upper case letter

pattern = ‘^[a-z]+_[a-z]+$’

Find sequences of one upper case letter followed by lower case letters.
import re

def text_match(S):

    pattern = '^[a-z]+_[a-z]+$'

    if re.search(pattern, S):
        return 'Found a match!'
    else:
        return('Not matched!')

# test
print(text_match("aab_cbbbc"))    # Found a match!
print(text_match("aab_Abbbc"))    # Not matched!
print(text_match("Aaab_abbbc"))   # Not matched!