A regular expression (regex or regexp for short) is a special text string for describing a search pattern. You can think of regular expressions as wildcards on steroids. You are probably familiar with wildcard notations such as *.txt to find all text files in a file manager. The regex equivalent is .*\.txt$.

There is a nice Regular expressions tutorial at http://www.regular-expressions.info/tutorial.html, which you can go through to learn more about Regex.

Ubuntu is a community developed, free and popular Linux distribution for desktops, laptops, and servers, forked from Debian and sponsored by Mark Shuttleworth. The name of the distribution comes from the African concept of ubuntu which may be rendered roughly as "humanity toward others", "we are people because of other people", or "I am who I am because of who we all are", though other meanings have been suggested.