Build and test JavaScript RegExp patterns with live matches, capture groups, replace preview, timing, flags, matchAll-friendly examples, and quick ReDoS risk hints.
Reviewed May 25, 2026. Privacy model: tool input is processed in your browser and is not uploaded to BytePane servers.
JavaScript regex answer
JavaScript regex can be written as a literal like /pattern/flags or constructed with new RegExp(pattern, flags). Use this tester when you need live JS RegExp matches, capture groups, named groups, replace() output, matchAll() examples, timing, and ReDoS hints before copying a pattern into frontend, Node.js, TypeScript, or Deno code.
If part of the pattern comes from a user search term, file path, package name, or other literal text, escape that value first with the RegExp Escape Tool before testing the final expression here.
Use test(), anchors, and tight character classes for form prechecks, then verify critical data server-side.
Use exec() or matchAll() when you need match indexes, numbered groups, or named groups.
Patterns copied from PCRE, Python, or regex101 should be checked in ECMAScript/JavaScript mode before production use.
Click building blocks to assemble a JavaScript RegExp, then test matches, groups, replacement output, and ReDoS risk below.
Builder tokens append to the current pattern. Use presets for full examples, then refine the expression with flags, test text, and replace preview.
Load a starter pattern, inspect matches and named groups, preview replace(), then read the caveat before shipping it.
Form validation precheck
/^[^\s@]+@[^\s@]+\.[^\s@]+$/Readable JavaScript email shape check for client-side validation before backend verification.
Caveat: Regex cannot prove mailbox ownership or deliverability; send a verification email for production accounts.
Find links in text
/https?:\/\/[^\s/$.?#].[^\s]*/gExtracts web URLs from logs, Markdown, docs, or pasted text using JavaScript global matching.
Caveat: Use the URL constructor for strict URL validation and normalization.
ID validation
/\b[0-9a-f]{8}-[0-9a-f]{4}-4[0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}\b/giMatches canonical UUID version 4 strings with the correct version and variant nibbles.
Caveat: This checks format only; it does not prove the ID exists in a database.
Code linting helpers
/^[A-Za-z_$][A-Za-z0-9_$]*$/Simple ASCII identifier check for scripts, generated code, config keys, and teaching examples.
Caveat: JavaScript supports Unicode identifiers too; this starter pattern intentionally keeps to ASCII.
Copy cleanup
/\b(\w+)\s+\1\b/giFinds repeated adjacent words and previews a JavaScript replace() cleanup with $1.
Caveat: For Unicode-heavy copy, prefer a Unicode-aware tokenization pass.
Controlled snippet extraction
/<a\b[^>]*\bhref=["']([^"']+)["'][^>]*>/giCaptures href values from simple anchor tags in controlled snippets and migration scripts.
Caveat: Do not parse arbitrary or malformed HTML with regex alone; use a DOM parser.
Paste a regular expression to check syntax, matches, capture groups, timing, and obvious ReDoS risks.
Test JavaScript replace() output with $&, $1, $2, and other numbered capture references before changing production text.
Replacement preview runs against all matches by adding the global flag when needed, so cleanup and redaction patterns show the full output.
Replacement output will appear here.
Copy the pattern into browser, Node.js, TypeScript, or Deno code using the right API for the job.
const re = /[A-Z]+/g;
const isMatch = re.test('Sample text 123');const re = /[A-Z]+/;
const match = re.exec('Sample text 123');
const groups = match?.groups ?? match?.slice(1);const re = /[A-Z]+/g;
const matches = [...'Sample text 123'.matchAll(re)];const re = /[A-Z]+/g;
const output = 'Sample text 123'.replace(re, 'replacement');// Modern Baseline 2025+ runtimes:
const re = new RegExp(RegExp.escape(userInput), '');
// Older runtime fallback:
const escapeLiteral =
RegExp.escape ??
((value) => value.replace(/[.*+?^${}()|[\]\\]/g, '\\$&'));
const compatibleRe = new RegExp(escapeLiteral(userInput), '');.Any character except newline
\dDigit (0-9)
\wWord character (a-z, A-Z, 0-9, _)
\sWhitespace (space, tab, newline)
^Start of string
$End of string
*0 or more
+1 or more
?0 or 1 (optional)
{n,m}Between n and m times
(abc)Capture group
(?:abc)Non-capturing group
[abc]Character class
[^abc]Negated class
a|bAlternation (a or b)
(?=abc)Positive lookahead
JavaScript regex patterns are created with /pattern/flags or new RegExp(pattern, flags). Use test() for a boolean check, exec() for one match with groups, matchAll() for all grouped matches, and replace() or split() when transforming text.
g global, i ignore case, m multiline, s dotAll, u Unicode, y sticky, d match indices, and v Unicode sets where supported.
Open the JavaScript regex guide for RegExp examples, then browse the regex pattern library for copy-paste validation, extraction, and cleanup patterns.
Most basic syntax transfers across engines, including character classes, anchors, alternation, capture groups, lookahead, and common quantifiers. The edge cases are where bugs appear: JavaScript uses $1 and $& replacement tokens, does not support recursive patterns, handles global matches through lastIndex, and supports modern flags such as d for indices and v for Unicode sets only in compatible runtimes.
If a pattern works in a multi-engine tester, switch that tester to ECMAScript or JavaScript mode, then paste it here with the exact flags and sample input your app will use. For structured formats such as JSON, HTML, URLs, CSV, or programming language syntax, regex is best for quick candidate extraction; a parser should own final validation.
The tester runs new RegExp(pattern, flags), match highlighting, replace preview, timing, and ReDoS hints in your browser tab. Tool input is not uploaded to BytePane servers. For sensitive logs, tokens, or customer data, still follow your team's policy before pasting into any web tool.
Regular expressions (regex) are the most powerful text pattern matching tool in computing. Every major programming language — JavaScript, Python, Java, Go, Rust, C# — supports regex natively. Originally developed by mathematician Stephen Kleene in the 1950s, regex is used billions of times daily for input validation, data extraction, search-and-replace operations, and log parsing.
Live
matches update in the browser as you edit the pattern, flags, or test string
Groups
capture groups are listed with match index so extraction patterns are easier to debug
ReDoS
common risky shapes are flagged before you copy a pattern into production code
Replace
preview search-and-replace output with capture references before running cleanup, redaction, or log parsing code
Email validation: Use ^[^\s@]+@[^\s@]+\.[^\s@]+$ as a readable front-line filter, then verify deliverability by sending email. Phone numbers: ^\+?[\d\s\-().]+$ is flexible enough for many user interfaces, but server-side code should normalize by country. URLs: https?://[^\s]+ is useful for extraction, while validation is often better handled with URL parsers. For a deeper copy-paste list, open the regex cheat sheet.
Catastrophic backtracking is the regex performance issue to check first. Nested quantifiers like (a+)+$ can cause exponential time complexity on adversarial input in backtracking engines. This is called ReDoS (Regular Expression Denial of Service). Always test regex patterns with edge cases before deploying to production. Use tighter character classes, anchors, timeouts, or a linear-time engine for untrusted input. BytePane's regex tester shows match timing and flags common risky pattern shapes.
Use regex for pattern matching, extraction, validation prechecks, and search-and-replace. Use a real parser for JSON, deeply nested HTML, CSV with quoted fields, programming language syntax, and URLs that need canonical normalization. Regex can quickly find candidate text; parsers should own structured formats and security-sensitive interpretation.
Use either a literal such as /pattern/flags or the RegExp constructor: new RegExp(pattern, flags). JavaScript regex supports common flags such as g, i, m, s, u, y, d, and v, plus methods like test(), exec(), match(), matchAll(), replace(), and split().
Use RegExp.escape(userInput) before embedding literal user text in new RegExp() when your runtime supports Baseline 2025 features. For older runtimes, use a vetted escape helper or polyfill, and never execute untrusted server-side regex patterns without review or a timeout.
Yes, but choose the ECMAScript or JavaScript flavor first and check engine-specific syntax. PCRE-only features, different replacement tokens, recursive patterns, branch reset groups, and some lookbehind behavior may not work the same in JavaScript RegExp.
Use test() for true or false validation, exec() for one match with capture groups, matchAll() for every global match with groups, replace() for text transforms, split() for tokenization, and search() when you only need the first match index.
JavaScript RegExp syntax overlaps with PCRE and Python for basics such as groups, alternation, anchors, character classes, and lookahead. Differences appear in engine-specific features, escaping rules, named groups, lookbehind support, Unicode behavior, and replacement syntax.
BytePane uses JavaScript's built-in RegExp engine. This is the same regex flavor used in Node.js, browsers, TypeScript, and Deno. Most patterns are compatible with PCRE or Python with minor differences.
Flags modify regex behavior. "g" finds all matches, "i" ignores case, "m" changes line anchors, "s" lets dot match newlines, "u" enables Unicode mode, "y" uses sticky matching, "d" returns match indices, and "v" enables Unicode sets in supporting engines.
Parentheses (abc) create a capture group that remembers matched text. You can reference numbered groups with $1 and $2 in replacements, or use named groups like (?<year>\d{4}) and $<year> when your JavaScript runtime supports them.
Yes. Enter replacement text under Replace Preview to transform every match in the test string. JavaScript-style replacement tokens such as $&, $1, $2, and $<name> are supported for common search-and-replace workflows.
No. The tester evaluates the pattern, flags, test string, replacement preview, and safety hints in your browser tab using JavaScript RegExp. Avoid pasting secrets into any web tool unless your team policy allows it.
No. The ReDoS checker is a fast browser-side heuristic for common risky shapes such as nested quantifiers, repeated broad wildcards, and repeated alternation. Production systems that accept user input should still use timeouts, review, or a linear-time engine where possible.