Valid phone numbers

#
# Given a text file file.txt that contains a 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.
# 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.

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