a regular expression (abbreviated regex or regexp) is a sequence of text characters.
a regular expression is a pattern describing a certain amount of text. Their name comes from the mathematical theory on which they are based.
Regular expressions are a language used for parsing and manipulating text. They are often used to perform complex search-and-replace operations, and to validate that text data is well-formed.
Today, regular expressions are included in most programming languages, as well as in many scripting languages, editors, applications, databases, and command-line tools.
Regular expressions are an extremely powerful tool for manipulating text and data. They are now standard features in a wide range of languages and popular tools, including Perl, Python, Ruby, Java, VB.NET and C# (and any language using the .NET Framework), PHP, and MySQL.