input: Write a function to check if a string contains only valid characters (lowercase letters, digits, and underscores).
  assert is_valid_string("valid_string123") == True
  assert is_valid_string("Invalid-String!") == False
  assert is_valid_string("another_valid_123") == True
output: def is_valid_string(s):
      for char in s:
          if not (char.islower() or char.isdigit() or char == '_'):
              return False
      return True