What do the regex flags (g, i, m, s, u) mean?

Flag 'g' (global) finds all matches rather than stopping at the first match. Flag 'i' (case insensitive) ignores capital and lowercase differences. Flag 'm' (multiline) causes ^ and $ to match the start and end of each line rather than the entire string. Flag 's' (dotAll) allows the dot (.) to match newline characters.

What is catastrophic backtracking in regex?

Catastrophic backtracking occurs when a regex engine attempts exponentially large numbers of combinations on non-matching strings, usually caused by nested quantifiers like (a+)+. It can freeze browser threads or crash backend servers.

What is the difference between capturing and non-capturing groups?

A capturing group (e.g. (abc)) memorizes the matched text for backreferencing ($1, $2). A non-capturing group (e.g. (?:abc)) groups characters together for quantification without allocating memory for backreferences, improving performance.

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Interactive Regex Tester & Pattern Generator

Test, debug, and inspect regular expressions with real-time match highlighting, pre-built pattern libraries, and instant code generation for JS, Python, and Go.

Presets:

2 matches foundEvaluated in 0.36ms

Test String114 chars

Match Details (2)

Match #1Index: 14 (Len: 20)

support@kitwharf.com

Match #2Index: 38 (Len: 31)

sales.inquiries@corp-global.com

Export Ready-to-Use Code Snippet

// JavaScript / TypeScript
const regex = /[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}/gi;
const text = `Contact us at support@kitwharf.com or sales.inquiries@corp-global.com for support. Invalid: user@.com, test@domain`;

// Match all occurrences
const matches = [...text.matchAll(regex)];
matches.forEach((match, i) => {
  console.log(`Match ${i + 1}: ${match[0]} at index ${match.index}`);
});

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Regular Expression (Regex) Quick Reference Cheat Sheet

Character Classes

. : Any character except newline
\d : Any digit [0-9]
\D : Any non-digit
\w : Word character [a-zA-Z0-9_]
\s : Whitespace (space, tab, newline)

Quantifiers

* : 0 or more times
+ : 1 or more times
? : 0 or 1 time (optional)
{3} : Exactly 3 times
{2,5} : Between 2 and 5 times

Anchors & Boundaries

^ : Start of string (or line in /m)
$ : End of string (or line in /m)
\b : Word boundary
\B : Non-word boundary

Groups & Lookarounds

(abc) : Capture group
(?:abc) : Non-capturing group
(?=abc) : Positive lookahead
(?!abc) : Negative lookahead

Frequently Asked Questions

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