What are regular expressions?

Regular expressions have been developed in the Unix environment in order to be able to have a flexible and powerful tool to search for strings in text files. They are based on the following idea: a set of commands allows to search for partial strings. The partial strings may be anything from a simple substring to a complex string derived from a template. These partial strings may be concatenated and combined by some extra logic to form even more complex strings.

Note: Beginners often have problems in using regular expressions due to the fact that both the commands ("expressions") and the data fed to the commands are plain ASCII characters. So it's important to be able to recognize the role of a particular character in a regular expression. More details can found in the section about regular expressions.