Regular expressions

Utilities offered by many programming languages for parsing text and extracting information based on a pattern. Based on concepts from theoretical computer science. Best suited for unstructured text; not to be used to parse HTML.

Cheatsheet

• https://regex101.com/
• https://pythex.org/

\s   whitespace
\S   non-whitespace

Python

(?:...)         non-capturing group
(?P<name>...)   named group
(?#...)         comment
(?!...)         negative lookahead assertion

• \n matches a literal newline. re.DOTALL causes . to also match newlines. re.MULTILINE doesn't affect newline matching; it causes ^ and $ to match line boundaries as well.

JavaScript

const pat = /([0-9]{4})-([0-9]{2})-([0-9]{2})/g;

pat.test("on 2025-05-04") // true
pat.exec("on 2025-05-04") // ["2025-05-04", "2025", "05", "04"]
pat.exec("abc") // null