Regex Cheatsheet

What is this for?

A printable, searchable summary of the bits of regex syntax you keep half-remembering. The tables here cover the major categories — anchors, character classes, quantifiers, groups, lookarounds, flags — plus a starter set of common patterns. Click any pattern to copy it; type in the filter to narrow down. Pair this with the Regex Tester to actually try the patterns against text.

When to use it

• You need (?<=foo) and can't remember whether ? goes before or after the <.
• You're explaining regex to someone and need a stable reference page rather than rummaging through Stack Overflow tabs.
• You want a starter pattern (UUID, email, ISO date) you can copy and tweak rather than write from scratch.
• You need to know which flag does what — particularly s (dotall) vs m (multi-line), which people routinely mix up.

Common gotchas

• Flavour matters. Most of this is JavaScript / modern PCRE, but features differ. Lookbehind only landed in JavaScript with ES2018; x (extended) is PCRE/Python and not in JS; possessive quantifiers ++ are PCRE-only.
• m ≠ "multi-line matching". m changes what ^ and $ mean (per-line instead of per-string). To match across line breaks with ., you want s (dotall).
• Greedy matches eat too much. <.*> against <a>b</a> matches the whole thing, not just <a>. Use <.*?> for the lazy version, or better yet a more specific class like <[^>]+>.
• Regex isn't an HTML or JSON parser. The "common patterns" here are good for one-off scraping or validation hints, not for treating structured input as a string.
• Email regexes are always wrong. The example here is a rough shape-check; for production validation, send a confirmation email instead.
• Don't trust copy-pasted "perfect" regexes. Test them against your real data with the Regex Tester before deploying.