Regular expressions are text patterns that define the form a text string should have. Using them, among other usages, it will be possible to do the following activities:
Check if an input honors a given pattern; for example, we can check whether a value entered in a HTML formulary is a valid e-mail address
Look for a pattern appearance in a piece of text; for example, check if either the word "color" or the word "colour" appears in a document with just one scan
Extract specific portions of a text; for example, extract the postal code of an address
Replace portions of text; for example, change any appearance of "color" or "colour" with "red"
Split a larger text into smaller pieces, for example, splitting a text by any appearance of the dot, comma, or newline characters
In this chapter, we are going to learn the basics of regular expressions from a language-agnostic point of view. At the end of the chapter, we will understand how regular expressions work, but we won't yet be able to execute a regular expression in Python. This is going to be covered in the next chapter. Because of this reason, the examples in this chapter will be approached from a theoretical point of view rather than being executed in Python.