123024.com · Regex Tester
Regex Tester is a free online tool for testing regular expressions with real-time match highlighting, capture group inspection, and replacement preview. Includes a built-in pattern library and token-level explanation. Runs entirely in your browser.
A regex (regular expression) tester is an interactive tool that lets you build and test regex patterns against sample text in real time. Use it alongside the JSON formatter for data validation or the Text Case Converter for batch text transformations.
As you type, matches are highlighted directly in the test text, and a detailed match list shows each match with its position, captured groups, and length.
This tool runs entirely in your browser — no data is ever sent to any server.
Real-time match highlighting — see matches visually in your test text with alternating colors for clarity.
Capture group inspection — view named and numbered capture groups for each match.
Replacement preview — test regex-based find-and-replace with support for $1,
amp;, and lt;name> replacement tokens.Pattern explanation — token-level breakdown of your regex, showing what each part means (e.g., \d+ → “one or more digits”).
Responsive matching — results update as you type, with a 1,000-match render cap so chatty patterns stay usable.
Preset library — quick-start with common patterns for email, URL, IP, phone, passwords, and more.
Learning regex — see how each token contributes to matching, and test your understanding against sample text.
Debugging — paste an existing regex and test it against actual data to verify it matches what you expect.
Data extraction — use capture groups to extract specific parts from log files, CSVs, or structured text.
Find and replace — preview complex text transformations before applying them in your code.
Validation — test email, phone, URL, and other validation patterns against edge cases.
Character classes: \d any digit, \w word character, \s whitespace, [...] custom class, [^...] negated class.
Anchors & boundaries: ^ start, $ end, \b word boundary, \B non-word boundary.
Quantifiers: * zero or more, + one or more, ? zero or one, {n} / {n,} / {n,m}, and lazy *? +? ??.
Groups & lookaround: (...), (?:...), (?<name>...), (?=...), (?!...), (?<=...), (?<!...).
Escapes: \n newline, \t tab, \\ backslash, \. literal dot, \/ literal slash.
The interactive cheatsheet above the guide lists each token with meaning and example.
JavaScript regex only — this tool uses the browser’s built-in regex engine. PCRE, Python, and other regex flavors may behave differently.
No lookbehind in older browsers — lookbehind assertions (?<=...) require ES2018+.
Match cap at 1,000 — to keep the UI responsive, only the first 1,000 matches are rendered.
Capturing groups not highlighted in text — groups are shown in the match list, not inline in the highlighted text.
Can I test regex with flags? Yes. This tool supports all standard JavaScript regex flags: g (global), i (case-insensitive), m (multiline), s (dotAll), u (unicode), y (sticky), and d (has indices).
What is the Explain mode? Explain mode breaks down your regex pattern into human-readable descriptions, showing what each token matches and how quantifiers and groups work.
Is there a match limit? Yes. To keep the UI responsive, only the first 1,000 matches are rendered. For patterns with more matches, the tool shows that the list is truncated.