phone number formatter to utilise regular expressions

[silaselisha]

if len(phone_number) < 9: raise IllegalPhoneNumberException('Phone number too short')
I believe the utility function for formatting phone numbers should be implemented using a regular expression, unless there was a specific reason not to do so

[Naftali-B]

Using regular expressions can be computationally more expensive (big O-notation) than simple string manipulation methods like checking the length and for non-alphanumeric characters.

When you're dealing with a specific and relatively simple phone number format, such as those commonly used in a particular region, using regular expressions might be unnecessary overhead. In such cases, directly checking the length and presence of non-alphanumeric characters can be more efficient.