Regex Tester

How Regex Tester works

Test regular expressions live with highlighted matches, capture groups and a replace preview. Everything happens inside this page: the tool reads your input, runs the calculation or conversion locally, and hands the result back to you. Nothing is transmitted, because there is no backend to transmit it to.

That design choice is deliberate. Most free tool sites upload your files to a server, process them there, and quietly keep a copy. Toolcin cannot do that — the site is a set of static files. If your connection dropped the moment the page finished loading, every tool would still work.

What the tester shows

Type a pattern, paste sample text, and every match is listed with its capture groups. The flags row controls how the engine behaves, and the replacement field previews a substitution using $1, $2 and the rest.

With the global flag on, every match is collected; with it off, only the first match is returned — which is exactly how the same pattern behaves in JavaScript, so the difference is visible rather than surprising.

The five flags, in plain terms

Flags change the meaning of the pattern, not just its speed. Mixing them up is the most common reason a regex “works on regex101 but not in my code”.

Greedy, lazy and catastrophic backtracking

A quantifier is greedy by default: .* takes as much as it can and then gives characters back until the rest of the pattern matches. Adding ? makes it lazy, so it takes as little as possible — the difference between matching a whole line and matching one tag.

Nested quantifiers such as (a+)+ are the classic way to hang an engine. On a short string they are fine; on a 10,000-character line they can take minutes. If a test seems to freeze, that is usually the cause, and the fix is almost always to anchor the pattern or replace nesting with a simpler expression.

Replace preview and why it matters

The replacement field runs String.replace with your pattern, so the preview shows the real output rather than an approximation. That catches the common mistakes early: forgetting that $& means the whole match, or that a literal dollar sign must be escaped as $$.

Special characters have to be escaped to be literal. A dot matches any character; \. matches a dot. A slash inside a character class is literal, but a slash in a URL pattern outside one needs escaping if you are writing a literal regex in code.

Good to know

• Works offline once loaded, including on flights and in locked-down networks.
• No file size limit beyond your device's own memory.
• Free to use, with no account and no usage counter.
• Results are produced locally, so they are not logged or shared with anyone.

Questions