Regular expressions are special kinds of tools used to represent patterns syntactically. When working with any type of textual input, you don't always know what the value will be, but you can usually assume (or even demand) the format you are going to receive into your application. These types of situations arise when you create a regular expression to extract and manipulate this input.

Consequently, to match a specific pattern requires a very mechanical syntax, since a change in even a single character or two can vastly change the behavior of a regular expression and, as a result, the final outcome as well.

Regular expressions by themselves (or Regex, for short) are not specific to any single programming language and you can definitely use them in nearly all the modern languages straight out of the box. However, different languages have implemented Regex with different feature sets and options; in this book, we will be taking a look at Regex through JavaScript, and its specific implementation and functions.