Free · Fast · Privacy-first

Test Regex Online, Live Match Highlighting

Q: Why test regex online instead of in my code?

Online testers let you iterate on patterns without modifying and re-running code. Every change to the pattern produces an immediate visual response: the match highlights update in real time, capture group values refresh in the results panel, and error messages appear for syntax mistakes. There is no file to open, no script to run, and no console output to parse. The turnaround from an idea to a verified result is measured in seconds. For debugging a failing pattern, the isolated environment also removes all surrounding application code from the equation, making the pattern-to-input relationship the sole focus.

Q: Does it support regex lookaheads?

Yes. JavaScript regex supports both positive lookaheads ((?=...)) and negative lookaheads ((?!...)), and both work in the FixTools tester. Lookbehinds were introduced in ES2018: positive lookbehinds use ((?<=...)) and negative lookbehinds use ((?<!...)). Both are supported in all modern browsers and work correctly in the tester. The match highlights show what the main pattern captures, while the lookaround content is asserted but not included in the highlighted match region, which is the correct behaviour for zero-width assertions.

Q: Can I test regex across multiple lines?

Yes. Enable the m (multiline) flag so that ^ and $ match the start and end of each individual line rather than the start and end of the full string. This is the setting you need for patterns that should match one record per line in a multi-line block of data. To make the dot (.) match newline characters so patterns can span across line boundaries, also enable the s (dotAll) flag. The two flags serve different purposes and can be used independently or together.

Q: What happens if my regex has an error?

If your pattern is syntactically invalid, the tool shows an error message immediately below the pattern field describing the problem, and all match highlights are cleared. The error message identifies the type of problem so you can correct it. Common errors that trigger this include unclosed parentheses or square brackets, an invalid quantifier range where the minimum exceeds the maximum such as {5,2}, unrecognised characters used as flags, invalid Unicode property names used with \p{} in Unicode mode, and escape sequences that are not valid in standard mode.

Q: Can I save my regex patterns?

You can bookmark the page URL, which updates in supported browsers to reflect your current pattern and flags, or copy the URL from the address bar to share or save a specific configuration. Pasting the URL in a new browser tab restores the pattern exactly. For patterns you use frequently across projects, consider storing them in a code snippet manager, a project-level constants file, or a shared team documentation page where colleagues can also access them.

Q: How do I match a pattern that appears anywhere in a string?

Simply omit the ^ and $ anchors from your pattern. Without anchors, the regex engine searches for the pattern starting at every position in the string and reports the first position where a complete match is found. Add the g flag to find every occurrence in the string rather than stopping at the first match. This is the standard behaviour for search and extraction use cases, as opposed to validation use cases where anchors are used to require the pattern to match the full string from start to finish.

Q: Can I test regex with Unicode characters?

Yes. Enter Unicode characters directly into the test string by pasting them or typing them with a Unicode-capable keyboard layout. For patterns that need to match Unicode properties rather than specific characters, use Unicode property escapes like \p{Letter} or \p{Script=Greek} and enable the u (unicode) flag in the flag selector. Unicode mode also ensures that characters outside the basic multilingual plane, such as emoji and some CJK extension characters, are treated as single code points rather than as two separate surrogate pair values.

Q: Is there a limit to how long my test string can be?

The tool does not impose a hard character limit on test string length. You can paste substantial blocks of text such as a full log file excerpt or a page of document content. However, patterns prone to catastrophic backtracking can cause the browser to slow down or become temporarily unresponsive when applied to long strings. If this happens, simplify the pattern by replacing nested quantifiers with more specific negated character classes. A well-constructed pattern runs in linear time relative to input length and processes large strings without any noticeable delay.

Q: Will the same pattern behave identically in Node.js, Bun, and Deno?

For the great majority of features, yes. Node.js, Bun, and Deno all expose V8 (or in Bun's case, JavaScriptCore) regex semantics that conform to the current ECMAScript specification. Named groups, lookbehinds, Unicode property escapes, and the s and y flags work the same way in each runtime. The differences appear at the leading edge: the ES2024 v flag for Unicode set notation reaches each runtime on its own schedule, and very large patterns can hit per-engine memory limits at different thresholds. If a pattern parses cleanly in the browser tester and uses only features available since 2018, you can safely assume the same pattern will run identically across all three major server-side JavaScript runtimes.

Q: How do I avoid creating a ReDoS vulnerability with user-supplied input?

ReDoS, or regex denial of service, happens when a pattern with catastrophic backtracking is run against an attacker-chosen input that forces the engine into exponential work. The fix has two parts. First, audit your patterns for the three risky structures: nested quantifiers like (a+)+, overlapping alternatives like (a|a)*, and unbounded repetition that lacks an anchor at the trailing end. Replace these with possessive-equivalent forms using negated character classes such as [^a]*a, which cannot backtrack into a previously consumed character. Second, never compile a regex from untrusted user input directly: if users can submit search patterns, run them through a timeout-protected worker or a known-safe subset language rather than passing raw strings to RegExp.

Skip the write-run-debug cycle entirely.

⚡

Instant live match highlighting

🔒

Supports all standard regex flags

✨

Capture group results shown inline

⚡

Works in browser, no installation needed

Cost: Free forever
Sign-up: Not required
Processing: In your browser
Privacy: Files stay local

FreeNo signupWhite-label

Add this Regex Tester to your website

Drop the Regex Tester into any page — blog post, product docs, intranet, school portal — with a single line of HTML. Your visitors get the full tool, processed entirely in their browser. No backend, no uploads, no signup.

Files stay 100% in the visitor's browser
Responsive — adapts to any container width
Free forever, no API key needed

Embed code

<iframe
  src="https://www.fixtools.io/developer/regex-tester?embed=1"
  width="100%"
  height="780"
  frameborder="0"
  style="border:0;border-radius:16px;max-width:900px;"
  title="Regex Tester by FixTools"
  loading="lazy"
  allow="clipboard-write"
></iframe>

<iframe
  src="https://www.fixtools.io/developer/regex-tester?embed=1"
  width="100%"
  height="780"
  frameborder="0"
  style="border:0;border-radius:16px;max-width:900px;"
  title="Regex Tester by FixTools"
  loading="lazy"
  allow="clipboard-write"
></iframe>

Attribution-friendly: a small "Powered by FixTools" link appears in the embed footer.

Quick Pattern Verification Without Leaving the Browser

The phrase "test regex online" describes one of the most common developer micro-tasks: verifying that a pattern actually matches what you think it does before embedding it in code. Regex syntax is notoriously dense, and a single misplaced quantifier or forgotten escape can cause a pattern to silently match the wrong text or fail to match at all. Historically, developers verified patterns by writing short scripts, running them, reading output, and iterating. That loop added cognitive overhead and forced context-switching away from the problem at hand. Online testers collapse that loop entirely by displaying match results as you type, in the same browser tab where you are working, without any file to create or command to run.

JavaScript's NFA-based regex engine evaluates patterns by exploring possible match paths through the input string. For each candidate position, it attempts to match the pattern from left to right, backtracking when a branch fails and trying the next alternative. Greedy quantifiers like .* try to consume the maximum number of characters first, then give characters back one at a time until a full match is found or all possibilities are exhausted. Lazy quantifiers like .*? do the opposite, starting as short as possible and extending only when required. Flags alter engine behaviour in fundamental ways: i disables case sensitivity in every character comparison, m makes ^ and $ match line boundaries instead of string boundaries, and s makes the dot metacharacter match newline characters that it would otherwise skip.

For quick pattern verification, the most productive workflow is to paste three to five representative examples of your target data, enable the g flag to see all matches simultaneously, and confirm that every example is highlighted correctly. Then paste two or three counterexamples that should not match and verify they produce no highlights. This compact process, taking under five minutes in most cases, catches the majority of pattern errors before they reach code review. If an example is not highlighted or a counterexample is unexpectedly highlighted, you have a precise, isolated reproduction of the bug with no surrounding application code to distract from it.

One subtlety that often surprises developers is the difference greedy and lazy quantifiers make in extraction tasks. A pattern like <.+> applied to "<b>hello</b><i>world</i>" greedily spans from the first opening angle to the last closing angle, producing one giant match covering everything between. Switching to <.+?> makes the quantifier lazy and yields four separate matches, one per tag. The visual highlighting in the tester makes this difference immediate and obvious in a way that reading the regex on paper rarely does. The same dynamic shows up in CSV parsing where a greedy quoted-field pattern accidentally swallows the separator between two fields. Use the live highlights to confirm your quantifiers are doing exactly what you intended before promoting the pattern into a script that processes a million rows.

MDN Regular Expressions guide PCRE specification

How to use this tool

💡

Type your regex pattern and paste your test string. Matches are highlighted in real time, and the match list updates as you edit the pattern.

How It Works

Step-by-step guide to test regex online, live match highlighting:

1
Open the Regex Tester
Navigate to the FixTools Regex Tester by clicking the tool link. The interface opens immediately in your browser with no login, installation, or configuration required. The pattern field, flag selector, and test string area are all visible on a single screen.
2
Enter your pattern
Type your regex pattern directly into the pattern input field. Enter the pattern without surrounding slash delimiters: just the expression itself. If your pattern includes backslash sequences like \d or \w, type them as single backslashes. The tool handles all necessary internal escaping automatically.
3
Set flags
Choose any needed flags from the flag selector adjacent to the pattern field. The g flag finds all matches in the test string rather than stopping at the first. The i flag disables case sensitivity. The m flag changes ^ and $ to match line breaks rather than the string start and end. The s flag extends dot to match newlines.
4
Add a test string
Paste the text you want to match against into the test string area. Use real data from your project where possible: log lines, form submissions, API responses, or file contents. Realistic test data exposes edge cases that contrived examples miss, and you can paste multi-line blocks to test boundary behaviour across line breaks.
5
See live results
Matches are highlighted in the test string immediately as you edit either the pattern or the test input. The results panel below lists every match with its position range, the matched text, and the value of each capture group. Edit the pattern and watch the highlights update in real time without any delay or page reload.

Real-world examples

Common situations where this approach makes a real difference:

Quickly checking a copied pattern from Stack Overflow

When you find a regex pattern in a forum answer or a code comment, pasting it into an online tester with your own sample data takes under a minute and immediately confirms whether the pattern handles your specific input format. This is essential because forum patterns are written against the original author's data and may silently fail on your edge cases involving Unicode characters, trailing whitespace, unusual punctuation, or line ending differences. A 30-second test prevents silently adopting a broken pattern that only fails in production.

Validating CSV field extraction patterns

Data engineers building CSV parsers use the online tester to verify that their field-extraction pattern handles quoted fields, fields containing commas, and empty fields correctly. Pasting a representative selection of CSV rows and watching which portions are highlighted by each capture group saves significant time compared to running a full Python or Node.js parsing script for each iteration. When the capture groups show the right fields in the results panel, the pattern is ready to move into the pipeline code.

Prototyping patterns for a code linter rule

When writing a custom ESLint or similar linter rule that uses regex to flag forbidden patterns in source code, developers prototype the detection pattern in the online tester against code snippets copied from real files. The visual match display makes it straightforward to confirm that the pattern flags the intended construct without also flagging similar-looking legitimate code nearby. Refining the pattern in the tester is far faster than editing the linter plugin code and running the linter against the test file for each change.

Testing search patterns for text editors

Advanced text editor find-and-replace features accept regex patterns. Before running a global search across an entire project with hundreds of files, developers test the pattern in the online tester to confirm it matches exactly the intended lines and nothing else. This is particularly valuable for patterns using word boundaries or alternation, where a small syntax difference produces a dramatically different and potentially damaging match set when applied at scale.

Pro tips

Get better results with these expert suggestions:

Paste multiple test lines to check boundary behaviour

Enter several representative lines of real test data in the input area and enable the m flag. This lets you observe directly how anchors (^ and $) behave at line boundaries versus at the boundaries of the full string, which is critical for patterns that process multiline log files, multi-record CSV exports, or any other data format where records are separated by newlines rather than processed one at a time.

Use alternation to test multiple valid formats at once

If your pattern needs to match several structurally distinct formats such as two different date styles or two different phone formats, put one format per line in the test string and enable the g flag. You can then confirm that every format variant is highlighted correctly in a single view, without switching between separate test runs. This also makes it easy to catch an alternation branch that accepts one format but accidentally excludes another.

Check your quantifier greediness with nested tags

If you are matching HTML-like structures, test with a string that contains two or more tag pairs of the same type. A greedy .* will span from the first opening tag to the last closing tag, consuming everything in between as a single match. Switching to a lazy .*? reveals whether you need a negated character class such as [^<]* for reliable tag content extraction that stops at the first closing tag.

Remove flags one at a time to isolate their effect

If a pattern behaves unexpectedly, toggle flags off one at a time and observe how the match highlights change with each removal. The i flag is a common culprit when a pattern unexpectedly matches or fails to match on mixed-case input. The m flag changes where ^ and $ match in ways that are not always obvious. Removing flags individually takes seconds and identifies the source of the behaviour far faster than reasoning about all flags simultaneously.

FAQ

Frequently asked questions