11. Creating Clean and Maintainable Code

Activity 11.01: Expanding Phone Number Matching Patterns to Accept Multiple Formats

Solution:

1. Observe that only the characters at the beginning of each pattern differ, but the last characters, XXX-XXXX, are the same in both patterns.
2. For the differing characters, for our regex to match either format correctly, you can specify the regex snippets that correspond to each format as alternate expressions of an alternation. Recall that alternations take the form of (expression1|expression2):

       ([2-9]\d{2}-|\([2-9]\d{2}\) )
   where Regex for XXX-\Regex for (XXX)

3. Combine it with the rest of the original regex to get the complete regex:

       ^([2-9]\d{2}-|\([2-9]\d{2}\) )\d{3}-\d{4}$

4. Remember that the original regex, \d{3}-\d{4}, matches XXX-XXXX, which is the same for both patterns. We only needed the alternation for the beginning parts of each pattern that differed. The ^ and $ characters...