About
The Regex Checker lets you write and test JavaScript regular expression patterns against any input text, with match highlighting that updates as you type — no button press needed. Every match is highlighted inline in the test string, and a details panel shows each match's index, length, full value, and the values of all capture groups, including named groups using the (?<name>...) syntax. All five JavaScript regex flags are available as toggles: g (global), i (case-insensitive), m (multiline), s (dotAll), and u (Unicode). A built-in cheatsheet covers the most common regex constructs — character classes, anchors, quantifiers, groups, and lookaheads — and each entry is clickable to insert the construct directly into your pattern field. Everything runs in your browser using the native JavaScript RegExp engine, so results are exactly what you will see in Node.js, Chrome, and other V8-based runtimes.
How to use
- 1 Enter your regular expression pattern into the Pattern input at the top — do not include the surrounding slashes.
- 2 Toggle the flags you need using the flag buttons: g (global — find all matches), i (case-insensitive), m (multiline), s (dotAll — dot matches newlines), or u (Unicode mode).
- 3 Type or paste your test string in the Test String panel — matches are highlighted live in green as you type.
- 4 Inspect the match details in the Matches panel: each match shows its position, length, and the value of every capture group.
- 5 For named capture groups, use the (?<name>...) syntax in your pattern — the panel labels each group by name.
- 6 Open the Cheatsheet panel and click any construct (e.g. \d+, (?=...), [a-z]) to insert it at the cursor in the Pattern field.
- What regex engine does this tool use?
- The tool uses JavaScript's native RegExp engine — the same one built into Node.js, Chrome, Firefox, Edge, and Safari. This means results are exactly what you will see when you use your regex in JavaScript or TypeScript code. If you are writing a regex for a different language (Python, Java, Go), be aware that syntax differences exist, especially for lookaheads, lookbehinds, and named group syntax.
- What regex flags are supported?
- All five JavaScript regex flags are supported: g (global — find all non-overlapping matches rather than stopping after the first), i (case-insensitive matching), m (multiline — ^ and $ match start and end of each line, not just the whole string), s (dotAll — the dot metacharacter matches newline characters), and u (Unicode mode — enables full Unicode matching and the \p{...} property escapes).
- How do named capture groups work?
- Named capture groups use the syntax (?<groupname>pattern). For example, (?<year>\d{4}) captures four digits and names the group "year". The Matches panel displays both the numbered index and the name for each captured group, so you can verify your naming logic without guessing which index corresponds to which group.
- How is this different from regex101.com?
- regex101.com is a powerful multi-flavour tool with detailed explanations. This tool focuses specifically on the JavaScript RegExp engine and prioritises speed and privacy: no data is sent to any server, there is no account required, and the clickable cheatsheet lets you build patterns without leaving the editor. It is optimised for developers who need a fast, private JavaScript regex scratchpad.
- Is my test string or pattern sent to a server?
- No. Pattern matching runs entirely in your browser using the native JavaScript RegExp engine. Neither your pattern nor your test string is transmitted anywhere, which makes it safe to test regexes against log lines, PII, or any other sensitive text.