Regex Tester

Test regular expressions with live match highlighting, groups, flags, and replace—runs locally, no upload.

Regex checker & validator — uses the same JavaScript RegExp engine as Node and browsers. Patterns and test text never leave your device.

Engine

JavaScript RegExp—same as Node.js and browsers

Global /g

Find all matches; required for replace-all

Privacy

Worker runs locally—patterns never uploaded

Regex playground

Pattern library

Quick examples
/pattern/g

$& = full match, $1 $2 = capture groups. Runs entirely in your browser.

See matches update instantly as you type.

Runs 100% locally — no data leaves your browser.

Enter a regex pattern above to see matches here.

Related developer tools

JSON ValidatorValidate JSON before extracting fields.JSON FormatterBeautify API responses.Text DiffCompare pattern output changes.
By Muhammad Abdullah Rauf · Founder, EverydayTools.proUpdated 2026-05-20

What is a regex tester?

A regex tester runs a JavaScript regular expression against sample text and shows matches, capture groups, flags, and replace output—entirely in your browser with no upload.

A regular expression (regex) is a pattern language for searching, validating, and transforming text. Developers use regex in logs, APIs, form validation, and editors—but small syntax errors cause silent failures. A regex tester lets you paste a pattern and test string, toggle flags (global, multiline, case-insensitive), and see highlighted matches with group details before shipping code.

This tool uses the same JavaScript RegExp engine as Node.js and browsers, so results match runtime behavior for most web apps. It is not PCRE/Python syntax—possessive quantifiers and some escape rules differ. Patterns and sample text stay on your device; nothing is sent to EverydayTools servers.

Pair with JSON Validator when extracting fields from payloads, or Text Diff when comparing pattern output across versions.

Quick answers

Concise answers for common searches — definitions, steps, and comparisons.

How do I match a literal dot in regex?

Escape it: \. matches a literal period. Inside [...] many specials are literal: [.*+] matches dot, star, or plus.

Greedy vs lazy quantifiers?

Greedy * + ? {n,m} take as much as possible. Lazy *? +? ?? {n,m}? take as little—useful between delimiters like <tag>...</tag>.

Is the regex tester private?

Yes. All matching runs in your browser; patterns and test text are not uploaded to servers.

How browser regex testing works

Patterns compile with new RegExp(pattern, flags) in an isolated Web Worker. Matches are collected with exec() in a loop when /g is set, with caps on match count and execution time to reduce ReDoS risk.

Formula

Match: RegExp.exec(testString) in loop
Replace: testString.replace(regex, replacement)

Limitations

  • JavaScript RegExp only—not PCRE, Python re, or Go syntax
  • Very large test strings may disable highlighting for performance
  • Not a security scanner—do not rely on regex alone for XSS or injection prevention

How to use Regex Tester

  1. Enter pattern and test string

    Paste your RegExp pattern and sample text—or load Email, URL, or other presets from the library.

  2. Set flags

    Enable global (g) for all matches, multiline (m) for line anchors, dotAll (s) so . matches newlines, and unicode (u) when needed.

  3. Review matches and groups

    Highlighted text shows each match; open match details for indices, numbered groups, and named groups.

  4. Test replace and share

    Optional replacement string previews substituted output. Copy a share link with pattern and flags encoded in the URL.

Regex Tester examples

Find all digits in a line

Input

Pattern \d+ on text "Order 42 costs $19.99"

Output

Matches 42 and 19, 99

With /g the engine finds every digit sequence; without /g only 42 matches.

Email preset smoke test

Input

Common email pattern vs alice+tag@sub.example.co.uk

Output

Match or no match with highlighted span

Validates practical format checks—not RFC-complete deliverability testing.

Who uses Regex Tester?

Common real-world scenarios where this tool saves time.

Validate email and URL patterns

Load common patterns, then test edge cases (subdomains, plus addressing, ports) before form validation code.

Parse logs and API responses

Extract timestamps, IDs, or log levels from sample lines—locally, without uploading production logs.

Debug replace and back-references

Preview replace output with $1/$2 or named $<group> before running sed or editor batch replace.

Learn pattern syntax

Use the pattern explainer to see what each token means while building complex lookaheads and character classes.

Workflow guides

Step-by-step chains that connect related tools for common tasks.

From API sample to validated pattern

  1. Paste a sample response line into the test string field.
  2. Validate JSON structure first in JSON Validator if the payload is JSON.
  3. Build and test your regex with /g and the groups panel.
  4. Compare old vs new pattern output in Text Diff when refactoring extractors.

Reference tables

Regex tester vs JSON tools

When to use each developer tool on EverydayTools.

ToolBest for
Regex TesterPattern match, groups, replace preview on arbitrary text
JSON ValidatorSyntax-valid JSON before parsing or formatting
JSON FormatterBeautify and minify JSON payloads
Text DiffCompare two text or code versions line by line

Common mistakes to avoid

Forgetting to escape . * + ?

Use \. for a literal dot. Outside a character class these operators are metacharacters.

Expecting PCRE-only features in JavaScript

No possessive ++ or atomic groups; use JavaScript-compatible patterns or test in the target language.

Advertisement

Frequently Asked Questions

Does this use JavaScript regex or PCRE?

JavaScript RegExp (ECMAScript) in your browser—the same engine as Node.js and front-end apps. PCRE/Python differ on possessive quantifiers, (?P<name>), and some escapes—re-test in your target runtime for production patterns.

What does the global flag (g) do?

With /g the engine finds every match and advances through the string. Without /g only the first match is returned—replace also affects only the first occurrence unless /g is set.

What are regex flags and what does each one do?

g = all matches; i = case-insensitive; m = ^ and $ per line; s = dot matches newlines; u = Unicode; d = match indices; v = unicode sets; y = sticky. Combine flags: /pattern/gim.

How do I test regex online?

Enter pattern and test text—matches highlight as you type. Turn on /g for all occurrences. Add a replacement to preview substituted text. Syntax errors show immediately with a clear message.

What is catastrophic backtracking (ReDoS)?

Nested quantifiers like (a+)+ can make the engine try exponentially many paths on non-matching input, freezing the tab. Avoid nested quantifiers on the same class; test long non-matching strings; this tool times out long runs in a worker.

Is my pattern and text uploaded?

No. Matching runs locally in a browser worker. Your patterns and test strings never leave your device.

Privacy, accuracy, and trust

Privacy

Patterns and test strings are evaluated in a browser Web Worker. Nothing is uploaded to EverydayTools—safe for logs containing tokens or personal data.

Accuracy

Uses the native JavaScript RegExp engine—the same semantics as Node.js and modern browsers for the flags you select.

For debugging and learning patterns—not a substitute for security review or production ReDoS audits on user-supplied regex.

Part of Developer Tools

Related Tools

More free tools for the same workflow.

Cron Expression Generator

Build and parse five-field cron expressions with human-readable descriptions and next-run previews.

Extract Domains from Plain Text

Paste notes, articles, or chat exports and get deduplicated website names (example.com). Tidy whitespace, copy or .txt download. No upload—runs in your browser.

Text Diff Tool

Compare two plain-text inputs line by line and highlight changes. Local diff for logs and copy—not JSON structure.

Advertisement

Reviewed by EverydayTools Editorial Team on 2026-05-20.