grep with regex for phone number

Question

I would like to get the phone numbers from a file. I know the numbers have different forms, I can handle for a single one, but don't know how to get a uniform regex. For example

1. xxx-xxx-xxxx

2. (xxx)xxx-xxxx

3. xxx xxx xxxx

4. xxxxxxxxxx

I can only handle 1, 2, and 4 together

grep '[0-9]{3}[ -]?[0-9]{3}[ -]?[0-9]{4}' file

Is there any one single regex can handle all of these four forms?

Answer 1

grep '(([0-9]{3})|[0-9]{3})[ -]?[0-9]{3}[ -]?[0-9]{4}' file

Explanation:

([0-9]{3}) three digits inside parentheses

| or

[0-9]{3} three digits not inside parens

...with grouping parentheses - (...) - around the alternation so the rest of the regex behaves the same no matter which alternative matches.