Regex Tester

What you can do with this regex tester

• Test JavaScript regular expressions live in your browser
• Toggle g, i, m, s, u, y, and d flags with clear explanations
• Highlight matches safely inside the test string
• Inspect capture groups and named groups with indexes
• Try replacements with $1, $&, and $<name> tokens
• Load common pattern examples with realistic sample text
• Export match analysis as JSON — copy or download locally
• Private by design: regex and test data never leave your device

What is a regular expression?

A regular expression is a pattern that describes a set of strings. Instead of searching for fixed text, you can express rules like “one or more digits,” “an email-shaped token,” or “a date in YYYY-MM-DD form.” Regex appears in validation, search, log parsing, refactoring, and text cleanup.

What is a regex tester?

A regex tester lets you try a pattern against sample text and see matches immediately. A useful debugger also shows indexes, capture groups, replacement output, and clear errors when the pattern is invalid — so you can iterate without writing a throwaway script.

How to test a regular expression

1. Enter your pattern in the Pattern field (flags stay separate).
2. Toggle flags such as global (g) or ignore case (i).
3. Paste a realistic test string that includes edge cases.
4. Review highlighted matches, group values, and indexes.
5. Optional: try a replacement string and copy the result.

JavaScript regex flags

Common regex patterns

The tool includes loadable examples for email, URL, IPv4/IPv6 (basic), phone numbers, dates, times, hex colors, usernames, whitespace, digits, UUIDs, domains, and more. Patterns labeled “basic” or “common validation” are practical approximations — not full RFC validators.

Regex quantifiers

Quantifiers control repetition: * (0+), + (1+), ? (0 or 1), {n}, {n,}, and {n,m}. Add ? after a quantifier for lazy matching.

Regex character classes

Shortcuts like \d (digit), \w (word character), \s (whitespace), and . (any character except newline unless DotAll) keep patterns short. Character sets [...] and negated sets [^...] define custom ranges.

Regex groups and capture groups

Capturing groups (...) store substrings for inspection and replacement. Non-capturing groups (?:...) group without storing. Named groups (?<name>...) label captures for clearer replacements with $<name>.

Lookahead and lookbehind

Assertions check surrounding text without consuming it: (?=...), (?!...), (?<=...), and (?<!...). They are powerful for password rules and contextual matches, but syntax and support can differ across languages — this tool follows JavaScript.

Common regex mistakes

• Forgetting to escape special characters like ., *, or +.
• Using incorrect anchors so the pattern matches more (or less) than intended.
• Confusing greedy and lazy quantifiers.
• Forgetting the global flag when you need every match.
• Building character classes that accidentally include or exclude dashes and ranges.
• Pasting PCRE/Python-only syntax into a JavaScript tester.

Regex flavor differences

This tool uses the JavaScript RegExp engine in your browser. Flavors such as PCRE, Python re, Java, .NET, Ruby, and Go differ in escapes, lookbehind rules, Unicode properties, and replacement syntax. A pattern that works elsewhere may fail here — and that is expected.

Frequently asked questions

What is a regular expression?

A regular expression (regex) is a compact pattern language for finding, validating, or replacing text. In JavaScript, patterns are executed by the built-in RegExp engine.

How do I test a regex?

Enter a pattern, choose flags, paste a test string, and review matches, indexes, and capture groups. Results update as you edit. You can also try replacements and load examples.

What does the g flag mean?

The g (global) flag finds all matches instead of stopping after the first. Without g, JavaScript typically returns only the first match.

What does the i flag mean?

The i (ignore case) flag makes matching case-insensitive, so Hello matches hello.

What is a capture group?

Parentheses (...) create a capturing group that stores the matched substring so you can inspect it or use it in replacements as $1, $2, and so on. (?:...) is non-capturing.

What is the difference between greedy and lazy quantifiers?

Greedy quantifiers like * and + match as much as possible. Lazy forms like *? and +? match as little as possible while still allowing the overall pattern to succeed.

What is a lookahead?

A lookahead asserts that text ahead matches (?=...) or does not match (?!...) without consuming characters. It is useful for password rules and conditional matching.

What is a lookbehind?

A lookbehind asserts that text behind the current position matches (?<=...) or does not match (?<!...). Support depends on the browser's JavaScript engine.

Why is my regex not matching?

Common causes include missing escapes for special characters, wrong anchors (^/$), needing the i or m flag, greedy quantifiers consuming too much, or using syntax from another regex flavor that JavaScript does not support.

Can regex cause performance problems?

Yes. Some patterns with nested ambiguous quantifiers can backtrack heavily and run slowly on certain inputs. This tool limits input size, caps rendered matches, and warns when execution takes unusually long.

Is this Regex Tester private?

Yes. Matching runs locally in your browser. Your pattern and test string are not uploaded to Toolshelf servers for processing.

Does this tool support JavaScript regex?

Yes. This is specifically a JavaScript RegExp tester. It does not claim PCRE, Python, Java, .NET, Ruby, or Go compatibility.

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