Regex Tester

Test regular expressions in real-time with instant matching, group highlighting, and a handy cheat sheet.

Pattern

//g

Test String

Matches0 matches

Regex Cheat Sheet

.

Any character except newline

\d

Digit (0-9)

\w

Word character (a-z, A-Z, 0-9, _)

\s

Whitespace (space, tab, newline)

^

Start of string

$

End of string

*

0 or more

+

1 or more

?

0 or 1 (optional)

{n,m}

Between n and m times

(abc)

Capture group

(?:abc)

Non-capturing group

[abc]

Character class

[^abc]

Negated class

a|b

Alternation (a or b)

(?=abc)

Positive lookahead

About Regular Expressions

Regular expressions (regex) are the most powerful text pattern matching tool in computing. Every major programming language — JavaScript, Python, Java, Go, Rust, C# — supports regex natively. Originally developed by mathematician Stephen Kleene in the 1950s, regex is used billions of times daily for input validation, data extraction, search-and-replace operations, and log parsing.

Regex Usage Statistics

93%

of developers use regex at least monthly (JetBrains Developer Survey, 2025)

Most searched developer reference topic on Stack Overflow

50+

Years of regex in production — from Unix grep (1973) to modern AI code assistants

Most Common Regex Patterns

Email validation: The simplified pattern ^[^\s@]+@[^\s@]+\.[^\s@]+$ catches 99.9% of invalid emails while being readable. The full RFC 5322 compliant regex is 6,300+ characters — impractical for most applications. Phone numbers: ^\+?[\d\s\-().]+$ handles international formats flexibly. URLs: https?://[^\s]+ is the pragmatic approach, while fully compliant URL validation requires 2,000+ character patterns.

Regex Performance Tips

Catastrophic backtracking is the #1 regex performance issue. Nested quantifiers like (a+)+$ can cause exponential time complexity, freezing your application for minutes on adversarial input. This is called ReDoS (Regular Expression Denial of Service). Always test regex patterns with edge cases before deploying to production. Use atomic groups or possessive quantifiers when available, and set timeout limits for regex execution in production systems. BytePane's regex tester shows match performance in real-time, helping you identify slow patterns before they cause issues.

Frequently Asked Questions

What regex flavor does this tool use?

BytePane uses JavaScript's built-in RegExp engine. This is the same regex flavor used in Node.js, browsers, TypeScript, and Deno. Most patterns are compatible with PCRE (PHP, Python) with minor differences.

What are regex flags?

Flags modify regex behavior. "g" (global) finds all matches. "i" (case-insensitive) ignores case. "m" (multiline) makes ^ and $ match line starts/ends. "s" (dotAll) makes . match newlines. "u" (unicode) enables Unicode mode.

How do capture groups work?

Parentheses (abc) create a capture group that remembers the matched text. You can reference groups by number ($1, $2) in replacements. Use (?:abc) for non-capturing groups when you need grouping without capturing.