Turning strings into structured things

We frequently need to find things in strings. We’ve learned 'a' not in "silly" and 'abc' in "crabcake" the like to find out whether a string is a substring of another, but it’s a very common programming task to take in a string and produce some kind of structured output. Here are some examples:

• Taking a URL and determining the protocol to use and domain to contact.

• Taking an email and determining the domain to contact and the user to send to.

• Turning a configuration file into a runtime configuration.

• Looking through DNA bases to find patterns (e.g., two A bases followed by anything followed by a T base).

• Loading Python code and running it.

In general, the process of taking a string and producing a structured output is called parsing. Parsing is an old, complex, and beautiful topic. This week, we’ll learn about the simplest possible parsing regime, called regular expressions, and see how we can use them to achieve simple parsing tasks.