Test a regular expression

The flags, and when to use each

The Flags field takes only these letters and drops anything else you type into it.

JavaScript regex, and how other flavors differ

Regular expressions look the same everywhere until they do not. This tester runs JavaScript's engine, so a pattern that works here works in browsers, Node.js, Deno and TypeScript. Before copying a pattern into another language, check these:

• Named groups. JavaScript writes (?<year>\d{4}); Python's re module uses (?P<year>\d{4}).
• Anchors. JavaScript has no \A or \Z for the very start and end of the text. Use ^ and $ without the m flag instead.
• Possessive quantifiers such as a++ (PCRE and Java) and recursion such as (?R) (PCRE) do not exist in JavaScript.
• Unicode classes such as \p{L} need the u flag in JavaScript, and Python's built-in re does not support them at all.
• Lookbehind, (?<=...), works in JavaScript at any length; some other engines allow only fixed-length lookbehind.

There is no PCRE or Python mode to switch to here. If a pattern is headed for PHP, grep or Python, test the parts these differences touch in that language as well.

Writing patterns that hold up

• Test what should not match. Paste near-misses next to the good examples: an email address with no dot after the @, a date with month 13. A pattern is only as good as the cases it turns away.
• Escape special characters you mean literally: . * + ? ^ $ { } ( ) | [ ] and the backslash itself. A bare dot in example.com matches any character, so it would also match examplexcom.
• Leave out the slashes. /abc/g is how code writes a regex literal. In Pattern type only abc, and put the g in Flags. A / needs no escaping here.
• Anchor when validating. To check that a whole value is a five-digit ZIP code, write ^\d{5}$, or 123456 will contain a match.
• Beware nested quantifiers. Patterns like (a+)+$ can take exponentially long on text that almost matches. If the tester stalls while you type one, a server running it would stall on the same input.

Matches of zero length, such as a lone ^ with the m flag, are counted but have nothing to highlight.

From testing to replacing

This tool finds and shows; it has no replace preview. Once the pattern is right, take it to Find and replace in Text Studio: switch on Pattern and it uses the same JavaScript regex, with $1, $2 in the replacement for your groups. That is how you reformat dates, swap first and last names, or clean up a list across a whole text. Common one-off jobs have tools of their own there too: pull out emails and links, remove duplicate lines and sort lines.

Regexes are usually tested on the very data they will run on: server logs with IP addresses and session IDs, exports of customer records, support emails. In Octoolo all of it stays on your computer while you work, and the tester needs no connection.