Valid Phone Numbers

Solution

Using egrep

egrep $=$ grep -E.

The difference is that -E option enables usage of extended regexp patterns. It will allow you using of such meta-symbols as +, ? or |. They aren't ordinary characters like we used to have in words or filenames but control commands for grep binary itself. Thus, | means logical OR.

egrep '^([0-9]{3}-|\([0-9]{3}\) )[0-9]{3}-[0-9]{4}$' file.txt

Using awk

awk '/^([0-9]{3}-|\([0-9]{3}\) )[0-9]{3}-[0-9]{4}$/' file.txt

Using sed

sed -r -n '/^([0-9]{3}-|\([0-9]{3}\) )[0-9]{3}-[0-9]{4}$/p' file.txt