Regex tricks

Programmers hate #2

Quickies

• [ -~] matches any ASCII printable character.
  Main limitation: only the ASCII range, won’t match the many Unicode letters outside of it.
• Matching delimited strings:
  • ('|")(\\?.)*\1 elegantly matches a single or double-quoted string with backslash escapes.
    YMMV depending on context, the first capturing group is used just for matching.
  • "(?:[^"\\]|\\.)*" matches a double-quoted string without using up a capturing group.
    Can alternatively use the simpler "([^"\\]|\\.)*" which does ; source.
• <(?:[^>"]|"[^"]*")*> can match ~tags (not their content) from simple subset of HTML, provided that a potential > that doesn’t terminate the tag is in a double-quoted attribute.
  The inspiration uses <([^>"]|"([^"\\]|\\.)*")*> but \ doesn’t escape attribute quotes in HTML.
  Mandatory reminder that arbitrary HTML can’t be matched with a regex.
• (?=(a+))\1 mimics a++ (possessive grouping) in flavors that don’t support it (say, JavaScript).
  Leaks capturing groups though. More details.

Matching exceptions to discard them

When trying to match some string except in some situations:

NotThis|NorThis|(ButThis)

Often simpler than trying to exclude via lookbehind, conditionals, parity checks, etc. Even \K — when using a compatible flavor, drops previous character from the match — becomes unwieldy when the exceptions are a bit complex.

It works by matching all the exceptions, and then only looking at the first matching group.

Downsides:

• Only works when you can discard matches.
• Matches on all exceptions, which can be expensive.
• The captured expression must not swallow exceptions.
• In JavaScript, requires the g flag and /…/g.exec(str)

Example usage in JavaScript:

const regex = /NotThis|NorThis|(ButThis)/g;
const matches = [];
let m;
while (m = regex.exec(str)) {
	if (typeof m[1] === "string") {
		matches.push(m[1]);
	}
}

Or with a generator function:

function* extract(str) {
	const regex = /NotThis|NorThis|(ButThis)/g;
	let m;
	while (m = regex.exec(str)) {
		if (typeof m[1] === "string") {
			yield m[1];
		}
	}	
}

For the full explanation and a good roundup of alternatives, see this article.