valid-phone-numbers.sh

#!/usr/bin/env bash
# Time:  O(n)
# Space: O(1)
#
# Given a text file file.txt that contains list of 
# phone numbers (one per line), write a one liner 
# bash script to print all valid phone numbers.
#
# You may assume that a valid phone number must 
# appear in one of the following two formats: 
# (xxx) xxx-xxxx or xxx-xxx-xxxx. (x means a digit)
#
# You may also assume each line in the text file 
# must not contain leading or trailing white spaces.
#
#    For example, assume that file.txt has the following content:
#
#    987-123-4567
#    123 456 7890
#    (123) 456-7890
#    Your script should output the following valid phone numbers:
#    987-123-4567
#    (123) 456-7890
#
#
# Read from the file file.txt and output all valid phone numbers to stdout.
# Using grep:
grep -P '^(\d{3}-|\(\d{3}\) )\d{3}-\d{4}$' file.txt

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

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