Regex tester

What it does

Write a regular expression and see straight away what it matches: every match is highlighted in your text and listed with its position, numbered groups and named groups. The pattern is also explained piece by piece in plain English, so a line like (?<year>\d{4})-(\d{2}) reads as “group 1, named year: a digit, exactly 4 times” instead of a puzzle. Replace mode shows the result of a search-and-replace with $1, $<name> and $& as you type. Matching uses your browser’s own JavaScript engine, the same one that runs your front-end and Node.js code, so what works here works there. A badly written pattern can take minutes to run on some text (catastrophic backtracking); here it’s stopped after 1.5 seconds with an explanation, instead of freezing the page.

How to use it

1. Type your pattern between the slashes, or start from one of the examples. Turn flags on or off below it: g finds every match, i ignores case, m makes ^ and $ work per line.
2. Paste the text to test. Matches are highlighted as you type, and each one is listed with its position and capture groups.
3. Read “What the pattern means” to check each part of your regex in plain English.
4. To replace, switch on “Replace” and write the replacement, using $1 or $<name> for groups. Copy the result, or copy the regex itself for your code.

Common uses

• Writing a validation pattern for a form field and checking it against good and bad examples
• Pulling dates, IDs or email addresses out of a log file or a pasted report
• Understanding a regex someone else wrote before changing it
• Testing a find-and-replace with capture groups before running it on a whole codebase

Tips

• Without the g flag, JavaScript stops at the first match. That’s what String.prototype.match and RegExp.exec do in your code too.
• Nested quantifiers like (a+)+ or (\w+\s?)* can backtrack exponentially on text that almost matches. Make each part match something distinct, or use a character class instead of alternation.
• Named groups, (?<name>…), make replacements readable: $<year>-$<month> is easier to check than $2-$1.