------
## 最终结论
```python
import re

def text_match(string):
    if re.fullmatch(r'[a-z]+_[a-z]+', string):
        return 'Found a match!'
    else:
        return 'Not matched!'

# Test cases
assert text_match("aab_cbbbc") == 'Found a match!'
assert text_match("aab_Abbbc") == 'Not matched!'
assert text_match("Aaab_abbbc") == 'Not matched!'
```