Start with two letters and followed by any number of Integers

regex for inline substitution w/ sed

When looking for day, Sunday should not be matched.

should start with letter follow by letters and numbers and (-) - zero or more can't start or finish with (-) can't have two or more consecutive (-) char

Match a not-empty string. A string with only spaces or no characters is an empty-string. When match something, mathc all the string with the exception of the eventually space at the and

White space

correctly handle the case where the value is splitted on multiple lines.

A powerful regex (fewest steps possible) which gives you all the information about urls.

Fully qualified domain name validation