How do I test a regex that needs to match across multiple lines?

Enable the m (multiline) flag to make ^ and $ anchors match the start and end of each line rather than the entire string. For patterns that need . to match newline characters as well, enable the s (dotAll) flag — without s, the dot matches any character except newline which makes multi-line span matching impossible. You can combine both flags (ms) for patterns that match across line boundaries and need line-start/end anchors.

Why does my regex match too much or too little?

Quantifiers like * and + are greedy by default, meaning they match as much text as possible before backtracking — this causes patterns like to match from the first rather than individual tags. Add a ? after the quantifier (.*? or .+?) to make it lazy (non-greedy), matching as little as possible. If the pattern matches nothing at all, verify that your flags are set correctly — a case-sensitive pattern will not match if the i flag is not enabled.

Are there security risks to testing user-supplied regex patterns?

Certain regex patterns can cause catastrophic backtracking, where the engine tries exponentially many combinations before reporting no match — this is called ReDoS (Regular Expression Denial of Service). Patterns like (a+)+ or (.+)* applied to long strings that almost but do not quite match can hang the browser tab for seconds or minutes. The tester runs in your browser so only your tab is affected, but be cautious about deploying user-supplied regex patterns in production server code without validation.

How do I match a literal dot, parenthesis, or other special character?

Escape special regex metacharacters with a backslash — to match a literal dot use \., to match a literal parenthesis use $ or $, and to match a literal backslash use \\. The full list of characters that need escaping in JavaScript regex is: . * + ? ^ $ { } [ ] | ( ) and \ itself. A common mistake is writing a pattern like 192.168.1.1 to match an IP address — without escaping, the dots match any character, so this pattern would match strings like 192x168y1z1.

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Regex Tester

Test regular expressions against sample text in real time. Supports flags and capture groups.

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Test String

How to Use

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Enter your regex pattern

Type a regular expression pattern and optionally set flags (g, i, m, s, u). Use the cheatsheet reference if you need help with regex syntax.

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Test against your text

Paste or type the text to test against. Matches are highlighted in real time as you type. Capture groups are shown individually for each match.

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Copy pattern or matches

Copy your finalized regex pattern or the matched results. Ideal for debugging regex before adding it to your production code.

What is Regex Tester?

Regex Tester is a browser-based tool for writing, testing, and debugging JavaScript regular expressions against live input text. It highlights all matches inline, displays capture group values, and shows a total match count so you can instantly see the effect of each pattern change. Developers, data engineers, and security analysts use this regex tool to build patterns for input validation, log parsing, and text extraction.

Why use Regex Tester?

Using JavaScript's native RegExp engine ensures the patterns you test behave identically in Node.js, browsers, and JavaScript-based serverless functions — no surprises when you move from testing to production. Real-time match highlighting with capture group breakdown makes it easy to iterate on complex patterns with named groups or lookaheads without repeatedly running test scripts. Supporting all JS regex flags (g, i, m, s, u) in the tool interface means you test exactly the flags your code will use.

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How it works

Backend developers use the regex tester to build and validate patterns for express-validator or Joi schema rules before embedding them in form validation logic. Data engineers craft log-parsing regex patterns against sample log lines from production before deploying a structured logging pipeline. Security engineers test input sanitization patterns against known attack strings like XSS payloads and SQL injection fragments to ensure their blocklist logic is exhaustive.

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Pro Tip

Use named capture groups with the (?<name>...) syntax when your regex extracts multiple pieces of data — the tester displays them by name making it far easier to verify extraction logic than numbered groups like $1 and $2. The s (dotAll) flag makes the . character match newline characters, which is essential for matching multi-line log entries or HTML blocks that the default mode misses entirely.

Frequently Asked Questions

Yes, the tester uses JavaScript's native RegExp engine which is identical across modern browsers and Node.js since they both implement the ECMAScript specification. This means patterns you test here will behave exactly the same in your Express.js route matching, React form validation, and browser-side input sanitization code without any engine-specific differences. However, if you are targeting Python, Go, or other languages, be aware that those use different regex engines with different syntax support.

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