Regex Tester — Test Regular Expressions
Test regular expressions with live matching and capture groups.
g = global, i = case-insensitive, m = multiline, s = dotAll
What to do next
Related tools
About Regex Tester
Regex Tester is a free, in-browser developer tool. Test regular expressions with live matching and capture groups. The page exposes a small surface — input, controls, output — so a first-time visitor can complete the job without reading documentation.
From a technical standpoint, Regex Tester is JavaScript and standard browser APIs running in your tab. The browser is the runtime; the page is the interface. Maximum input size: 0 MB per run.
Regex Tester performs the transformation entirely inside the JavaScript runtime. Your file lives in the tab's memory while the engine works on it; the result lives in the tab's memory until the browser triggers the download. Both are released when the tab closes, the way every browser tab releases its memory.
Typical users of Regex Tester include students learning new languages, backend developers inspecting requests and engineers debugging API payloads. The thread connecting all of them is the same: a focused developer utility task that fits cleanly into a browser tab and benefits from a tool with sensible defaults and minimal setup.
The right moment to reach for Regex Tester is when you have a focused developer utility job that fits inside a browser tab. Open the page, drop in the file or paste your input, choose the options that matter, and the tool returns the result.
On limits: 0 MB per file is the ceiling. Output formats and quality settings are listed in the controls panel above, and they apply to every run.
Regex Tester sits in a small group of related tools. Useful neighbours include Find & Replace, JSON Formatter, Diff Checker, and URL Encoder / Decoder. They are designed to compose: the output of one is a sensible input to the next, so a multi-step task is usually a sequence of single-click operations.
The transformation in Regex Tester is deterministic — the same input plus the same options produces the same result every run. That predictability matters when the result has to match an upstream specification or be reproducible later.
The download is delivered as a clearly named file the moment processing completes — no email link, no "your result will be ready in 5 minutes" queue, no expiry timer. The file is generated in your browser and saved by your browser's normal download flow.
Some context on why Regex Tester exists in this form: modern File APIs, high-performance JavaScript engines, and well-maintained open-source libraries now make it possible to perform developer utility work entirely in the browser. Regex Tester is built on top of that capability, which is why a single page can host the full pipeline.
Regex Tester produces deterministic output: the same input plus the same options always produces the same result. That predictability matters when the result has to match an upstream specification or be reproducible later.
A few practical tips that experienced users of Regex Tester pick up over time. First, keep your default browser updated — the engine relies on standard web APIs and newer browser versions are noticeably faster than ones from a few years ago. Second, close other heavy tabs before processing a large input; the engine shares CPU and memory with whatever else is open. Third, if you re-run the same kind of job often, your last-used settings are remembered for the rest of the tab session, so subsequent runs are essentially one click.
For most failure modes, refreshing the page and re-running the job is enough — the engine has no persistent state to corrupt. If the same input fails twice in a row, the input itself is most likely the problem (a truncated file, an unexpected variant of the format, or a stream the engine does not recognise).
Regex Tester is intentionally narrow in scope so the common case is fast and the result is predictable. If you ever need a variation it does not cover, browse the rest of the catalog — there is a good chance an adjacent tool already exists, and switching between tools is just a matter of opening another tab.
How it works
- 1Reach the Regex Tester page in your browser to begin.
- 2Drop a developer file onto the upload area, or click to pick one from your device.
- 3Adjust the options to match what you need. Sensible defaults cover the most common case, so you can usually skip this step.
- 4Hit the run button. standard browser APIs does the work in your browser tab.
- 5Download the result. The file is generated in your browser and saved through your normal download flow.
- 6Run additional jobs as needed. The same controls and defaults apply on every run.
Common use cases
- Pretty-print a minified blob during incident triage using Regex Tester.
- Inspect a regex against a test string before committing it.
- Convert between data formats while wiring up an integration.
- Decode a token to confirm its claims during a debugging session.
- Format a noisy log line into something a teammate can read.
- Generate boilerplate from a single specification line.
- Compare two API responses to spot a regression.
- Validate a config blob before pushing to staging.
FAQ
Which regex flags are supported?
Global (g), case-insensitive (i), multiline (m), and dotAll (s) flags are available.
Can I see capture groups?
Yes — named and numbered capture groups are displayed for each match.
Does it show the match count?
Yes — total match count and individual match positions are displayed.
Does Regex Tester support batch processing?
Regex Tester processes one input at a time by design — it keeps memory usage predictable on lower-end devices and makes results easier to verify. To handle a folder, run the tool once per file; the page stays loaded between runs and remembers your last-used settings, so the second run is essentially instant.
Is there a programmatic version of Regex Tester?
Regex Tester is a browser-only tool by design and does not expose a hosted API. The reason is the same as the privacy story: there is no Favtoo backend doing the work, so there is no service to call. If you need to script the same transformation, the underlying engine (standard browser APIs) is open-source and can be used directly from your own code.
How long does Regex Tester take to process a file?
Most jobs finish in seconds. Speed scales with input size and with how many CPU cycles your browser tab has available — the engine runs in your browser, so it shares resources with whatever else you have open. For inputs near the 0 MB ceiling, expect anywhere from a few seconds to roughly a minute on a typical laptop. Closing other heavy tabs noticeably speeds things up.
Can I use Regex Tester on iOS or Android?
Regex Tester runs in any modern mobile browser — Safari, Chrome, Firefox and the in-app browsers in most messaging apps all support the underlying APIs. Performance depends on the device: a recent phone handles typical inputs nearly as fast as a laptop, while older devices may take a few seconds longer near the 0 MB ceiling. The interface lays out cleanly on small screens, so you do not need to pinch-zoom to see the controls.
How do I know I am using the latest version of Regex Tester?
Regex Tester is updated whenever the underlying engine releases an improvement or a bug fix. Because the tool is delivered as a static page, every visit fetches the latest version automatically — there is no "version" to manage on your end. If a particular release ever changes default behaviour, the change is documented on Favtoo's changelog so you can confirm what shifted.
Which file formats does Regex Tester accept?
The accepted formats are listed in the upload area on the tool itself. If your input is in a format that is not directly supported, convert it first using one of Favtoo's converter tools — every Favtoo converter outputs a file that is a clean input to the next tool in the chain.
How many times per day can I use Regex Tester?
Inputs are capped at 0 MB per file, which keeps memory usage stable across phones, tablets and older laptops. You can run Regex Tester as often as you need; every run produces a full-quality result.
Does Regex Tester match what professional tools produce?
Regex Tester is built on standard browser APIs, which is the same class of engine used by professional developer utility pipelines. For deterministic operations, the output is byte-identical to what an equivalent CLI run would produce; for operations involving a codec or a model, the result is well within the range of what comparable tools generate. If you have a specific reference output you need to match, run a small test job first to confirm the configuration produces what you expect.