The regular expression, or regexp, are the most powerful, versatile and hated tool used by programmers and system administrators.
They allow to express with a few characters search for strings, characters or words, and if done well can lead to good results, but if they are wrong they can not give you any useful result, and the worst thing is that often it is difficult to understand whether or not a regepx it is written with a correct syntax to cover all the possibility.
But now as first thing let’s see what is a regular expression:
From WIkipedia ”
In computing, a regular expression, also referred to as regex or regexp, provides a concise and flexible means for matching strings of text, such as particular characters, words, or patterns of characters. A regular expression is written in a formal language that can be interpreted by a regular expression processor, a program that either serves as a parser generator or examines text and identifies parts that match the provided specification.