Regex Tester

How to use the Regex Tester tool

1. Type the pattern into the box between the two slashes. Leave out the slashes themselves, because the page already draws them.
2. Set the flags in the small box after the pattern. The tool always adds g, because without it you would only see the first match.
3. Paste the text to test against into the left panel. Matches are highlighted as you type.
4. Read the Matches panel for the details: each match with its position, every numbered capture group, and any named groups.
5. A red message under the pattern box means the pattern itself is invalid. It shows the browser's own error text, such as "nothing to repeat".

How to use a regular expression in JavaScript, PHP and Python

const re = /(?<user>[\w.+-]+)@(?<domain>[\w-]+\.[\w.]+)/g;
const text = "billing@acme.com and finance+archive@acme.co.uk";

// Every match, with groups
for (const m of text.matchAll(re)) {
    console.log(m[0], m.groups.user, m.groups.domain);
}

// Just a yes or no
console.log(/^\d{4}-\d{2}-\d{2}$/.test("2026-09-10")); // true

// Replace with a callback
console.log(text.replace(re, (_, user) => user + "@example.com"));

<?php
$re = '/(?P<user>[\w.+-]+)@(?P<domain>[\w-]+\.[\w.]+)/';
$text = 'billing@acme.com and finance+archive@acme.co.uk';

preg_match_all($re, $text, $matches, PREG_SET_ORDER);
foreach ($matches as $m) {
    echo $m[0], ' -> ', $m['user'], ' at ', $m['domain'], PHP_EOL;
}

// preg_match returns 1, 0 or false. false means the pattern failed,
// which is not the same as "no match", so check it explicitly.
if (preg_match('/^\d{4}-\d{2}-\d{2}$/', '2026-09-10') === 1) {
    echo "looks like a date\n";
}

import re

pattern = re.compile(r"(?P<user>[\w.+-]+)@(?P<domain>[\w-]+\.[\w.]+)")
text = "billing@acme.com and finance+archive@acme.co.uk"

for m in pattern.finditer(text):
    print(m.group(0), m.group("user"), m.group("domain"))

# fullmatch anchors both ends without needing ^ and $
print(bool(re.fullmatch(r"\d{4}-\d{2}-\d{2}", "2026-09-10")))

# Always use a raw string, or every backslash needs doubling
print(re.sub(pattern, lambda m: m.group("user") + "@example.com", text))

Common errors and how to fix them

The pattern matches far more than expected

Quantifiers are greedy: .+ matches as much as it can and backs off only as far as it must. Add a question mark for the lazy version (.+?), or replace the dot with a character class that cannot cross the boundary, such as [^<]+.

SyntaxError: Invalid regular expression: nothing to repeat

A quantifier has nothing before it. This is usually an unescaped + or ? that was meant as a literal character. Escape it as \+ or \?.

The page or the server hangs on a long input

This is catastrophic backtracking. A quantifier inside a quantifier, like (a+)+ or (\s*\w+)*, has an exponential number of ways to split a string that fails to match. Restructure the pattern so each character can be consumed only one way, or add an anchor that fails early.

^ and $ do not match at each line

Add the m flag. Without it, ^ and $ mean the start and end of the whole string, not of each line.

The pattern works here but not in PHP or Python

Regex flavors differ. JavaScript has no \A, \z or possessive quantifiers, Python spells named groups (?P<name>...), and Go's RE2 has no backreferences or lookaround at all. Test the tricky parts in the target language.

Good to know