Regex Tester

Regex Tester

Test JavaScript regular expressions against sample text in real time. Enter a pattern, choose optional flags, and inspect match counts, indexes, matched text, and capture groups without leaving your browser.

How to use this regex tester

1. Enter a JavaScript-compatible regular expression, such as \\bcat\\b.
2. Paste sample text into the test string field.
3. Enable flags when needed:

Flag	Purpose
g	Find every match. This tester enables global matching automatically.
i	Ignore letter case.
m	Make ^ and $ work at line boundaries.
s	Let . match newline characters.
u	Enable Unicode code point behavior.

Invalid syntax appears immediately with the JavaScript engine error. A valid pattern reports zero matches when no text matches.

Common regular expressions

Use case	Pattern
Whole word	\\bword\\b
Email address	^[^\\s@]+@[^\\s@]+\\.[^\\s@]+$
URL	https?://[^\\s]+
US phone number	\\(?\\d{3}\\)?[-.\\s]?\\d{3}[-.\\s]?\\d{4}
ISO date	\\d{4}-\\d{2}-\\d{2}

Patterns should match the actual input format. For production validation, combine regular expressions with type checks and business rules instead of relying on one expression for every requirement.

Capture groups and match indexes

Parentheses create capture groups. For example, (?<user>[^@]+)@(?<domain>[^@]+) separates an email into user and domain portions. Match details include each full match, its zero-based index in the test string, and captured values.

const expression = /(?<user>[^@]+)@(?<domain>[^@]+)/g;
const text = "ada@example.com";
const match = expression.exec(text);

console.log(match?.groups?.user);   // ada
console.log(match?.groups?.domain); // example.com
console.log(match?.index);          // 0

JavaScript example

const pattern = new RegExp("\\bcat\\b", "gi");
const matches = [..."A cat and another CAT".matchAll(pattern)];

console.log(matches.map((match) => match[0]));
// ["cat", "CAT"]

Python comparison

JavaScript and Python share many regex concepts, but syntax and supported flags differ. Always test patterns in the same language and runtime used by your application.

import re

pattern = re.compile(r"\bcat\b", re.IGNORECASE)
matches = pattern.findall("A cat and another CAT")
print(matches)

Tips for reliable patterns

• Escape punctuation when matching literal characters.
• Prefer bounded quantifiers such as {1,20} when input length is known.
• Use non-capturing groups (?:...) when you do not need a captured value.
• Test empty input, multiline input, Unicode characters, and unexpected punctuation.
• Avoid nested unrestricted quantifiers that can cause excessive backtracking.