What's the difference between greedy and lazy quantifiers?

Greedy (*, +, ?) match as much as possible. Lazy (*?, +?, ??) match as little as possible. Use lazy when matching between delimiters: instead of .

When should I use a non-capturing group?

Use (?:...) when you need grouping for alternation or quantifiers but don't need the captured value. It's faster and keeps capture indices clean.

Why doesn't my multiline regex work?

By default ^ and $ match the start/end of the string, not each line. Add the m flag for line-based anchors. For . to match newlines, add the s (dotall) flag.

Should I parse HTML/JSON with regex?

No. Use a real parser (DOMParser, json.loads, etc.). Regex breaks on nested or malformed input. Use it for token-level extraction inside already-parsed text.

// regex

Regex Cheatsheet

The pieces you actually use — character classes, anchors, quantifiers, groups, lookarounds, flags — plus a recipe book of common patterns.

Updated 2026-04-28

Character classes

Token	Matches
.	Any character except newline.
\d	Digit [0-9]. \D = non-digit.
\w	Word char [A-Za-z0-9_]. \W = non-word.
\s	Whitespace (space, tab, newline). \S = non-whitespace.
[abc]	Any of a, b, c. [^abc] negates.
[a-z]	Range. Combine: [A-Za-z0-9_-].
\b	Word boundary. \B = non-boundary.

Anchors

Anchor	Matches
^	Start of string (or line in multiline mode).
$	End of string (or line in multiline mode).
\A	Absolute start of string.
\Z	Absolute end of string.

Quantifiers

Quantifier	Meaning
*	0 or more (greedy). Use *? for lazy.
+	1 or more. +? for lazy.
?	0 or 1. ?? for lazy.
{n}	Exactly n times.
{n,}	n or more.
{n,m}	Between n and m.

Groups & lookarounds

Construct	Meaning
(abc)	Capturing group. Refer back as \1 or $1.
(?:abc)	Non-capturing group. Use when you only need grouping.
(?<name>abc)	Named capture. Refer as \k<name> or $<name>.
(?=abc)	Positive lookahead — must be followed by abc.
(?!abc)	Negative lookahead.
(?<=abc)	Positive lookbehind.
(?<!abc)	Negative lookbehind.
a\|b	Alternation: a or b. Group with (?:a\|b) to scope.

Flags

Flag	Effect
i	Case-insensitive.
g	Global — find all matches (JS/PCRE).
m	Multiline — ^ and $ match line boundaries.
s	Dotall — . matches newlines.
x	Extended — ignore whitespace and # comments in pattern (PCRE/Python).
u	Unicode (JS, Python).

Common recipes

Pattern	Regex	Note
Email (pragmatic)	^[\w.+-]+@[\w-]+(\.[\w-]+)+$	Don't try to be RFC 5322 perfect.
IPv4	^((25[0-5]\|2[0-4]\d\|1?\d?\d)\.){3}(25[0-5]\|2[0-4]\d\|1?\d?\d)$	Strict octet bounds.
URL (loose)	https?://[\w.-]+(?:\.[\w.-]+)+[\w\-._~:/?#[\]@!$&'()+,;=]	Use a parser if you can.
UUID v4	^[0-9a-f]{8}-[0-9a-f]{4}-4[0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}$	Case-insensitive flag recommended.
ISO 8601 date	^\d{4}-\d{2}-\d{2}(T\d{2}:\d{2}:\d{2}(\.\d+)?(Z\|[+-]\d{2}:?\d{2})?)?$	Date or datetime.
Strip ANSI color codes	\x1b\[[0-9;]*m	Useful for cleaning log output.

FAQ

What's the difference between greedy and lazy quantifiers?: Greedy (*, +, ?) match as much as possible. Lazy (*?, +?, ??) match as little as possible. Use lazy when matching between delimiters: <.+?> instead of <.+>.
When should I use a non-capturing group?: Use (?:...) when you need grouping for alternation or quantifiers but don't need the captured value. It's faster and keeps capture indices clean.
Why doesn't my multiline regex work?: By default ^ and $ match the start/end of the string, not each line. Add the m flag for line-based anchors. For . to match newlines, add the s (dotall) flag.
Should I parse HTML/JSON with regex?: No. Use a real parser (DOMParser, json.loads, etc.). Regex breaks on nested or malformed input. Use it for token-level extraction inside already-parsed text.