Regex Tester & Debugger
Test JavaScript regular expressions with match highlighting, flags, offsets, and capture groups.
Debug JavaScript regular expressions against sample text with match offsets, numbered and named capture groups, safe DOM highlighting, and a dedicated worker timeout for runaway backtracking.
Use Regex Tester & Debugger
The dedicated Web Worker was terminated to protect your browser responsiveness. This usually indicates catastrophic backtracking (ReDoS) caused by nested quantifiers (e.g. (a+)+$) or ambiguous repetitions.
Note: Worker termination protects UI responsiveness by cancelling the background thread, not that regex is inherently safe in all environments. Do not deploy expressions causing ReDoS to production servers.
Test String
Match Highlight Canvas
Capture Group & Offset Breakdown
| # | Range | Full Match | Capture Groups |
|---|---|---|---|
| No matches yet. Enter a valid pattern and test text. | |||
Built for the task, not the page view
- JavaScript flags g, i, m, s, u, and y
- Numbered and named capture groups with match offsets
- Dedicated Web Worker execution with a 200ms responsiveness timeout
- Zero-length match safeguards and a 5,000-match cap
- Email-like, IPv4 extraction, and ISO date extraction examples
JavaScript Regex PatternTest TextMatch HighlightsCapture Group TableMatches JSONUse it correctly
JavaScript vs. PCRE regex semantics
This tool evaluates regular expressions using your browser’s native ECMAScript RegExp engine (ECMA-262). JavaScript regex does not support PCRE-specific features like atomic grouping (?>...), recursive patterns (?R), branch reset groups (?|...), or POSIX character classes like [:digit:]. Always verify against JavaScript syntax if deploying to Node.js or browser environments.
Catastrophic backtracking (ReDoS) protection
Nested quantifiers like (a+)+ or ambiguous alternations can trigger exponential backtracking when evaluating non-matching input. To protect your browser tab from freezing, this tool runs evaluation in a dedicated Web Worker with a strict 200ms timeout. Terminating the worker preserves UI responsiveness, but does not make a dangerous regex safe for server-side use.
Named capture groups & match offsets
Modern JavaScript supports named capture groups using (?<name>expression). When matched, these groups appear under match.groups in JavaScript code. This tester displays start and end character offsets, numbered group indices, and named group labels for every match.
Lookahead & lookbehind compatibility
Lookahead assertions (?=...) and (?!...) are supported across all modern browsers. Lookbehind assertions (?<=...) and (?<!...) were introduced in ES2018; modern Chromium, Firefox, and Safari 16.4+ support them, but older WebKit/iOS versions may throw syntax errors.
In-browser privacy & analytics isolation
All regular expression compilation and matching execute inside a dedicated Web Worker in your local browser tab. Neither your regex pattern nor your test corpus is sent to an EveryTools transformation API or analytics service. Analytics record only fixed UI event identifiers, never user-entered text.
Common Pattern Starters & Practical Snippets
These starter patterns demonstrate real-world JavaScript regular expression idioms.Notice: These snippets are practical extraction and matching examples, not rigorous RFC or calendar validators.
Email-like matching (example)
Extracts user and domain parts using named capture groups. Practical for sanity-checking form inputs.
(?<user>[a-zA-Z0-9._%+-]+)@(?<domain>[a-zA-Z0-9.-]+\.[a-zA-Z]{2,})IPv4 address extraction (example)
Captures four dot-separated numeric octets using word boundaries.
\b(?<octet1>\d{1,3})\.(?<octet2>\d{1,3})\.(?<octet3>\d{1,3})\.(?<octet4>\d{1,3})\bISO 8601 date & timestamp (example)
Extracts YYYY-MM-DD dates with optional time and UTC/offset indicators.
(?<year>\d{4})-(?<month>0[1-9]|1[0-2])-(?<day>0[1-9]|[12]\d|3[01])(?:T(?<time>(?<hours>[01]\d|2[0-3]):(?<minutes>[0-5]\d):(?<seconds>[0-5]\d)(?:\.\d+)?)(?<timezone>Z|[+-][01]\d:?[0-5]\d)?)?Catastrophic Backtracking & ReDoS Prevention
Regular Expression Denial of Service (ReDoS) occurs when a regular expression engine enters exponential backtracking. When a pattern contains nested quantifiers or overlapping alternations (such as (a+)+$ or (a|a)+$), testing against an almost-matching string like "aaaaaaaaaaaaaaaaaaaa!" requires the backtracking engine to test millions of permutations before failing.
Why Web Worker isolation matters
In standard web applications, running regular expressions on the main UI thread blocks the JavaScript event loop. An exponential backtracking expression freezes the entire browser tab, preventing user clicks, scrolling, and page responsiveness. EveryTools runs all arbitrary regex evaluation inside a dedicated background Web Worker.
The 200ms timeout safeguard
When a regex evaluation starts, the client sets an unyielding 200ms timer. If the Web Worker does not produce a completed match result within 200 milliseconds, the main thread terminates the worker immediately viaworker.terminate() and alerts the user.
JavaScript ECMAScript RegExp Engine Reference
This debugger complies strictly with standard ECMAScript (ECMA-262) RegExp specifications.
| Flag | Name | ECMAScript Semantics |
|---|---|---|
g | Global | Finds all matches across the entire text rather than stopping after the first match. |
i | Ignore Case | Enables case-insensitive character matching for ASCII and Unicode ranges. |
m | Multiline | Changes ^ and $ to match the start and end of each line, not just the entire string. |
s | DotAll | Allows the dot (.) metacharacter to match newline characters (\n, \r). |
u | Unicode | Enables full Unicode code point handling and enables \p{...} property escapes. |
y | Sticky | Matches only from the index indicated by the lastIndex property in the target text. |
Metacharacter Cheat Sheet
| Token | Meaning | Example |
|---|---|---|
^ / $ | Beginning / end of string (or line with m flag) | ^Every |
\b / \B | Word boundary / non-word boundary | \btool\b |
\d / \D | Digit [0-9] / non-digit | \d{3} |
\w / \W | Word character [a-zA-Z0-9_] / non-word character | \w+ |
\s / \S | Whitespace / non-whitespace | \s*=\s* |
(?<name>...) | Named capture group (accessible via match.groups.name) | (?<id>\d+) |
(?:...) | Non-capturing group (groups tokens without capturing in match array) | (?:https?|ftp) |
(?=...) / (?!...) | Positive / negative lookahead assertion | \d+(?=px) |
(?<=...) / (?<!...) | Positive / negative lookbehind assertion (ES2018+) | (?<=\$)\d+ |
Frequently Asked Questions
What are the differences between JavaScript and PCRE regex?
Perl Compatible Regular Expressions (PCRE) is widely used in PHP, Apache, and C tools, offering features absent in ECMAScript: atomic groups ((?>...)), possessive quantifiers (*+, ++), recursive expressions ((?R)), subroutine calls, and branch reset groups ((?|...)). JavaScript RegExp engines run native ECMAScript semantics. Do not use PCRE-specific syntax in JavaScript code.
How do named capture groups work in modern JavaScript?
Introduced in ES2018, named capture groups use the syntax (?<groupName>subpattern). When an expression matches, the results object includes a groups dictionary mapping group names to their matched strings. This tester extracts both numbered indices ($1, $2) and named keys along with start and end character offsets.
How does this tester prevent browser tab freezing?
By delegating evaluation to a dedicated Web Worker off the main rendering thread, the browser UI remains completely responsive. A hard 200ms timeout cancels any run that takes too long. If an expression backtracks indefinitely, the background worker is immediately killed with worker.terminate() and a ReDoS diagnostic warning is displayed.
Does EveryTools store or inspect my regular expressions or sample data?
No. All regex evaluation and test string processing happen exclusively inside your browser's Web Worker. No regex patterns, test strings, or matched results are transmitted to any server. Our analytics system records only fixed interaction events (such as viewing the page or clicking "Test"), with zero payload content.