# Regex Debugger > What the Regex Debugger does, who it's for, and how it tests patterns live — your starting point for the manual, examples, and tips. Source: https://www.jpkc.com/db/en/tools/regex/ ## Regular expressions without the guesswork Regular expressions are powerful but unforgiving: one misplaced character and your pattern matches nothing — or suddenly far too much. The [Regex Debugger](https://www.jpkc.com/tools/regex/) takes the guesswork out of it. You type your **regex pattern** into one field and your **test text** into the other, and the tool shows you instantly what matches: every hit highlighted in color inside the text, plus a list with position and all capture groups. There's no "Run" button to wait for — evaluation happens **in real time** as you type. Edit the pattern and the matches follow along. That immediate feedback is what makes the difference: while you build, you see whether `\d{3}` actually fires, whether your **capture group** grabs the right thing, and exactly where your pattern gets too greedy. The tool is built for everyone who works with patterns: **developers** building and testing an expression for validation, search, or replacement; **admins and DevOps** picking apart log lines or config values; and anyone learning regex who needs something visible to react against instead of simulating it in their head. Everything runs in the browser — no account, no installation. ## What the tool does — at a glance The interface is two columns: pattern, test text, and matches on the left, a built-in cheat sheet on the right. In detail: - **Pattern** — the editor for your regex pattern. That's the bare expression, without the surrounding slashes; you pick the flags separately. - **Flags** — six toggles for `g`, `i`, `m`, `s`, `u`, and `y`. `g` (global, all matches) is on by default. - **Test String** — the input for the text your pattern runs against. Multi-line, with line numbers. - **Matches** — the results list: a counter ("N matches"), and for each hit the matched text, its **position** in the string, and all numbered and named capture groups. - **Highlighting** — every match is also marked in color directly in the test text (four rotating colors so consecutive hits stay distinguishable). - **Copy pattern** — copies your pattern with flags in literal format `/pattern/flags` to the clipboard. - **Clear all** — empties both editors and resets the flags to the default (`g`). - **Error display** — if your expression is syntactically invalid, the engine's original error message appears instead of a match list. - **Cheat Sheet** — a collapsible reference (Characters, Quantifiers, Groups, Anchors, Lookaround, Character Classes, Flags) right next to the editor. ## Which engine: native JavaScript Important for working accurately: the debugger uses your browser's native **JavaScript `RegExp` API** — not PCRE, not the Python or `.NET` variant. Your pattern is evaluated internally as `new RegExp(pattern, flags)`. That has two consequences: - You test the **exact engine** that will later run in your JavaScript, Node.js, or TypeScript code. What matches here matches there. - You need to know the **JavaScript limits**: the six flags `g/i/m/s/u/y`, named groups (`(?…)`), and lookahead **and** lookbehind are available. PCRE specialties such as atomic groups, possessive quantifiers, recursion, or POSIX classes (`[[:alpha:]]`) are not. More on this in the [manual](https://www.jpkc.com/db/en/tools/regex/manual/#engine-and-limits) and the [tips](https://www.jpkc.com/db/en/tools/regex/tips/#javascript-regexp-is-not-pcre). ## Try it now **[→ Open the Regex Debugger](https://www.jpkc.com/tools/regex/)** — type a pattern, paste test text, read the matches live. No account, free, right in the browser. The tool starts with a ready-made example (`(?\w+)` on a short sentence) so you can see immediately how matches, position, and a named group are displayed. ## Related JPKCom tools Regex is rarely an end in itself — it usually sits inside a larger text or code workflow: - **[Coder](https://www.jpkc.com/db/en/tools/coder/)** — encodes and decodes HTML entities, URLs, Base64, JWT, and data URIs; handy before or after you slice strings with regex. - **[Source Viewer](https://www.jpkc.com/db/en/tools/source/)** — displays and highlights source code in over 100 languages; great for finding the spot your pattern should target. - **[JSON Editor](https://www.jpkc.com/db/en/tools/json/)** — formats and validates JSON; often the cleaner choice than a regex when you need to take structured data apart. - **[Beautify](https://www.jpkc.com/db/en/tools/beautify/)** — formats JavaScript, CSS, and HTML; helps make a long string readable before you point a pattern at it. --- There's more on the subpages: the **[manual](https://www.jpkc.com/db/en/tools/regex/manual/)** with every feature and the engine limits in detail, **[examples](https://www.jpkc.com/db/en/tools/regex/examples/)** with runnable walkthroughs, and a collection of **[tips & tricks](https://www.jpkc.com/db/en/tools/regex/tips/)**.