Regex Tester

What Regex Tester does

Regex Tester runs a JavaScript regular expression against sample text and shows every match highlighted in place. The pattern, flags, test text and an optional replacement are all editable, and the results update as you type.

It is built for the fiddly part of writing regular expressions: checking that a pattern matches the right things and that it does not match the wrong ones. Paste a log line, a list of email addresses, a snippet of HTML or test data, then refine the pattern and watch the highlights and match list change. A pattern such as “\b\w+@\w+\.\w+\b” over “Contact [email protected] or [email protected]” reports two matches, at indexes 8 and 27.

The match counter shows how many matches were found, each row in the match list shows the matched text, its starting index and any captured groups, and adding a replacement previews the substituted result. Everything runs on the browser's regular expression engine.

How to use Regex Tester

1. Enter the pattern in the Pattern field and the text to search in Test text.
2. Tick the flags you need: g (global, on by default), i (ignore case), m (multiline), s (dot matches newlines) and u (Unicode).
3. Read the highlight — every match is marked in the text, or only the first without the g flag — and check the match count above it.
4. The match list below shows each match's text, starting index and capture groups; the first 20 are listed while the counter shows the total.
5. Type a replacement in Replace with to preview the result, using $1, $2 and $& in the same way JavaScript's replace does.
6. Click Copy result to copy the replaced text — it works once a replacement has been entered.

Features

• Live testing: the pattern, flags, test text and replacement all re-run as you type.
• Five flags: global, ignore case, multiline, dotAll and Unicode.
• Match highlighting plus a numbered match list showing matched text, index and capture groups, with the first 20 matches listed.
• Replacement preview using JavaScript's substitution patterns, with a copy button.
• Invalid patterns show the engine's own error message instead of failing silently.
• Zero-length matches are handled safely, so a pattern such as “a*” advances through the text instead of looping forever.

Tips and common mistakes

• The tester uses JavaScript's engine. Patterns written for Python, PCRE, Java or .NET can differ: possessive quantifiers and some inline modifiers are not standard JavaScript regex.
• There is no timeout or worker around the matching loop. A pattern with severe backtracking — nested quantifiers like “(a+)+” are the classic example — can freeze the tab on unlucky input, so test suspicious patterns on short strings first.
• The g flag controls both the highlighting and the replacement preview: without it, only the first match is found and only the first match would be replaced.
• Empty matches are legitimate. “a*” over “bab” reports four matches — an empty match at position 0, “a” at position 1, and empty matches at positions 2 and 3.
• Watch invisible characters. A pattern searching for a space will not match a tab or a non-breaking space, which is a common cause of “it should match but does not”.

Frequently asked questions

Related text tools: Find & Replace, Text Diff Checker, Line Tools, Word Counter.

🔒 Patterns and test text are processed in your browser; nothing is uploaded.