”I just need to match HTML tags”#

Almost every front-end developer has fallen into this trap. You just want to use regex to extract the content inside <div> tags:

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const html = '<div>First layer <div>Nested content</div></div>'
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const match = html.match(/<div>(.*)<\/div>/)
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console.log(match[1]) // "First layer <div> nested content" — unexpected!

Regular expressions are greedy by default. Add ? to switch to non-greedy mode? Still doesn’t work — nested tags are a context-free grammar, and regex, being a finite state automaton, theoretically cannot handle them perfectly.

The correct answer has long been enshrined in a legendary Stack Overflow response (which received over 4,000 upvotes):

You can’t parse [X]HTML with regex. Because HTML can’t be parsed by regex. Regex is not a tool that can be used to correctly parse HTML.
—— Stack Overflow user bobince

The original answer is thousands of words long, with a tone so impassioned it reads like the late-night cry of a tormented programmer. If you haven’t read it, it’s worth five minutes of your time.

”Validating date formats is simple”#

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// You thought that was enough
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const dateRegex = /\d{4}-\d{2}-\d{2}/
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dateRegex.test('2026-02-30') // true — February 30th?
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dateRegex.test('2026-13-01') // true — the 13th month?
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dateRegex.test('0000-00-00') // true — the dawn of chaos?

Regex only checks format, not semantics. Want to fully validate date legitimacy with regex? Congratulations, you’ll need to handle leap years, months with different lengths, and the historical quirk of October 1582 missing 10 days.

Regex matchmaking#

There’s also a classic Reddit post: someone used a regular expression to search for matches on the OkCupid dating site. His search criteria were:

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(?:engineer|developer|programmer|coder|hacker) (?!.*(?:politics|religion|vegan|gluten))

Translation: find an engineer/developer/programmer, but not one who talks about politics, religion, vegetarianism, or gluten-free diets all day.

How did it work out? The post was never updated, so most likely — now he has three problems.