<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def valid(self, name):
 """Ensure a variable name is valid. Note: Assumes variable names are ASCII, which isn't necessarily true in Python 3. Args: name: A proposed variable name. Returns: A valid version of the name. """

    name = re.sub('[^0-9a-zA-Z_]', '', name)
    if re.match('[0-9]', name):
      name = '_' + name
    return name