Regex Tester

Test regular expressions with real-time matching and highlighting

The Regex Tester is a browser-based regular expression tool that highlights matches in real time, supports match and replace modes, accepts standard flags (g, i, m, s, u), and includes one-click presets for common patterns like email, URL, and IP address.

Mode

Regular Expression

/ /

Test String

Matches

Replace Result

Frequently Asked Questions

How do I test a regular expression against sample text online?

Enter your pattern in the regex field between the two slash delimiters, set any flags (g for global, i for case-insensitive) in the flags field, then type or paste your test string in the text area below. Matches are highlighted in real time as you type. The match list below shows each match, its index, and its character position.

Is this regex tester safe to use with sensitive data?

Yes. The Regex Tester runs entirely in your browser using the JavaScript RegExp engine. No data is transmitted to any server. You can safely test patterns against real log lines, PII, or internal data without it leaving your machine.

How do I use the regex replace mode to test substitutions?

Switch the mode toggle from Match to Replace. A replacement string field appears below the test text area. Enter your replacement expression (you can use $1, $2 for capture groups), and the replaced output is shown live. This is useful for validating find-and-replace operations before applying them in code or an editor.

What are the quick-pattern presets and when should I use them?

The preset buttons (Email, URL, IP Address, Phone, Hex Color, Date) load commonly needed regex patterns with appropriate flags in one click. Use them as a starting point and modify the pattern to match your specific format. They are not guaranteed to cover every edge case — for instance, the email preset follows a common approximation, not the full RFC 5321 specification.

Why does my regex work in JavaScript but not in Python or other languages?

This tool uses the JavaScript RegExp engine, which differs from PCRE (used in PHP, Python’s re module, and many others) in several ways. JavaScript does not support lookbehind in older engines, uses different syntax for named groups (?), and handles Unicode differently. Always test in the target language before deploying a pattern to production.