What are Regular Expressions?

Regular Expressions are a formal, or syntactical, language, used to create strings of text, which define a search pattern. The writer of a regular expression has the choice to create a regular expression as specific or as relaxed as he wishes, meaning that it could match only a very specific piece of text, or it could possibly match many text strings, as long as all of them follow the same pattern described by the regular expression. Many programs and programming languages contain a RegEx processing engine, which allows the programmer to define patterns, rather than specifying exact strings for everything. A regular expression is a piece of text that contains regular characters, known as literals, and special characters, known as non-literals. You can think of non-literals as 'commands' that tell the engine to do something special. For example, the regular expression tal[cekly] means something that starts with tal and ends with either c, e, k, l or y. We are using just 10 characters to describe 5 distinct words!