Valid Phone Numbers

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

关键的地方在于使用正则表达式组匹配行首，注意只有ERE才支持正则表达式组，awk默认是ERE，grep 中使用参数-E指定ERE，sed使用-r指定ERE

# Read from the file file.txt and output all valid phone numbers to stdout.
#grep -E '^(([0-9]{3}) |[0-9]{3}-)[0-9]{3}-[0-9]{4}$' file.txt
#awk '/^(([0-9]{3}) |[0-9]{3}-)[0-9]{3}-[0-9]{4}$/' file.txt
sed -r -n '/^(([0-9]{3}) |[0-9]{3}-)[0-9]{3}-[0-9]{4}$/p' file.txt