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.

Attached with this blog is the RegularExpression.pdf, that contains the quick reference sheet on how to write the Regex patterns.