Regular Expressions

Regular expressions (or regexes) are a domain-specific programming language, defining a grammar for expressing efficient and flexible string comparisons. Introduced in 1951 by Stephen Cole Kleene, regular expressions have become a popular tool for searching and manipulating text. As an example, if you're writing a text editor and you want to highlight all web links in a document and make them clickable, you might search for strings that start with HTTP or HTTPS, then those that contain ://, and then those that contain some collection of printable characters, until you stop finding printable characters (such as a space, newline, or the end of the text), and highlight everything up to the end. With standard Python syntax, this will be possible, but you will end up with a very complex loop that will be difficult to get right. Using regexes, you match against https?://\S+.

This section will not teach you the full regular expression syntax — there are...