------
## 最终结论
```python
import re

def text_lowercase_underscore(s):
    pattern = r'^[a-z]+_[a-z]+$'
    if re.match(pattern, s):
        return 'Found a match!'
    else:
        return 'Not matched!'

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