Synctoolo
Developer Tools

Regex Tester

Build, test, and debug regular expressions with real-time match highlighting, full capture group breakdown, vetted pattern presets, and string substitution. Runs 100% in your browser using standard ECMAScript regular expression evaluation.

Regex Tester2 matches found
Presets:
Toggle flags to modify engine behavior
//
Flags:
(135 chars, 1 line)
Editor
Visual Match Highlights2 matches
Contact our support team at support@synctoolo.com#1 or sales-inquiries@partner.org#2. Avoid sending to invalid@host without a tld or test@.

Match Inspector & Capture Groups

2 matches
#1support@synctoolo.com
pos: 28..49 (len: 21)
Group $1:support
Group $2:synctoolo.com
#2sales-inquiries@partner.org
pos: 53..80 (len: 27)
Group $1:sales-inquiries
Group $2:partner.org

How to use Regex Tester

Intuitive, friction-free workflow designed to get you from input to accurate results in seconds.

01Step 1

Input or Paste

Type or paste your regular expression between the forward slashes.

02Step 2

Instant Calculation

Toggle regex flags (g for global, i for case-insensitive, m for multiline, s for dotAll, u for Unicode).

03Step 3

Copy & Use Results

Enter your test string or choose a vetted preset like Email, URL, IPv4, or UUID.

04Step 4

Copy & Use Results

Inspect highlighted matches in Split or Highlights view to verify character spans.

05Step 5

Copy & Use Results

Review the Match Inspector table to verify numbered and named capture groups.

06Step 6

Copy & Use Results

Optionally open the Regex Replace panel to test string substitution using $1, $2, or custom replacement tokens.

Core Capabilities

Engineered for speed, privacy, and browser-first accuracy

Real-time visual match highlighting with individual match counters
Vetted pattern library for emails, URLs, IPv4, UUIDs, hex colors, dates, and slugs
Interactive flag toggles with instant engine re-evaluation
Detailed capture group inspector showing start/end indices and group values
Regex substitution panel supporting $1, $2, $&, and custom text replacements
Infinite loop protection for zero-length pattern matches
Client-side execution ensuring your test data never leaves your computer

Common Scenarios

Everyday workflows for writers, developers, and professionals

Validating user input formats for forms, APIs, and CLI utilities
Extracting tokens, timestamps, or IP addresses from server and application logs
Testing complex capture groups before embedding patterns in JavaScript or TypeScript codebases
Performing automated search-and-replace text sanitization before database insertion
Learning regular expression syntax with immediate visual feedback
Client-Side Privacy · Zero Paywalls

Why use Synctoolo's Regex Tester?

Many online regex testers run obsolete regex libraries or send your text to remote servers. Synctoolo evaluates your pattern natively in your browser engine with zero server latency, zero data logging, and complete support for modern JavaScript RegExp features.

100% Free Forever

Zero paid tiers, hidden subscriptions, or paywalls

Zero Account Walls

No forced email signups or credit meters

Local Memory Safe

Processed inside your browser, never stored in DB

Instant Processing

Runs locally in browser memory without cooldowns

In-Depth Guide: Regex Tester

Authoritative standards, practical benchmarks, and best practices.

How to test regular expressions before production

Regular expressions power search-and-replace, form validation, log parsing and web scraping. A regex that works on three test cases often fails on the fourth because of edge cases: empty strings, Unicode characters or greedy matching.

A live tester shows every match highlighted in your sample text. You see capture groups numbered so you know which part of the pattern matched which substring. This is faster than console.logging RegExp.exec() in a loop.

Regex flags and common pitfalls

The global (g) flag finds all matches, not just the first. The case-insensitive (i) flag treats A and a as equal. Multiline (m) changes how ^ and $ behave across lines. Without the right flags, your pattern silently returns fewer matches than expected.

Catastrophic backtracking can freeze browsers on badly designed patterns with nested quantifiers. If your tester hangs, simplify the pattern or use atomic groups. Test against realistic input lengths, not just three-word strings.

Frequently Asked Questions

Straightforward answers regarding regex tester calculations, platform limits, and data privacy.

Which regular expression engine does this tester use?+

This tool runs directly on the ECMAScript RegExp engine built into your web browser (such as V8 in Chrome, JavaScriptCore in Safari, or SpiderMonkey in Firefox). Patterns verified here will behave identically in Node.js, Deno, Bun, and modern browsers.

What do the regex flags (g, i, m, s, u) mean?+

The 'g' flag finds all matches rather than stopping at the first. The 'i' flag ignores uppercase versus lowercase distinctions. The 'm' flag treats start (^) and end ($) anchors on each line individually. The 's' flag allows the dot (.) character to match newline characters. The 'u' flag enables full Unicode code-point matching.

How does regex substitution and replacement work?+

Expand the 'Regex Replace & Substitution' section and enter your replacement pattern. You can use standard tokens like $1 or $2 for capture groups, $& for the entire matched string, or fixed replacement text. The preview updates instantly as you type.

Can I inspect named capture groups?+

Yes. If your regular expression uses named capture groups such as (?<year>\d{4}), the Match Inspector displays both the group name and its captured value alongside numbered groups.

Is my test data private?+

Yes. All regex execution, match calculation, and substitution take place entirely client-side inside your browser memory. No text or regex patterns are ever transmitted to any external server or saved in any database.

Why does my pattern cause an error?+

If your pattern has unescaped special characters, unclosed parentheses, or unsupported quantifiers, the tool highlights the pattern field in red and displays the exact syntax error message reported by the engine.

Guides & Tutorials for Regex Tester

In-depth articles and tutorials to get the most out of Regex Tester.

Developer•8 min read

Regex for Email Validation: Why RFC 5322 Patterns Fail in Practice and Safe Practical Solutions

Learn how to validate email addresses with regular expressions. Understand the official RFC 5322 standard, catastrophic backtracking risks, and practical regex patterns.

Read guide
Developer•10 min read

Catastrophic Backtracking in Regular Expressions: How ReDoS Breaks Production Servers

Stop Regular Expression Denial of Service (ReDoS). Discover why nested quantifiers cause exponential execution times and how to test patterns safely.

Read guide
Developer•11 min read

20 Essential Regex Patterns Every Developer Needs (With Interactive Testing Guide)

A practical developer cheat sheet for regular expressions. Learn the syntax, avoid catastrophic backtracking, and test 20 battle-tested patterns for email, URL, phone, and password validation.

Read guide
Developer•10 min read

Why You Should Never Paste API Keys, Passwords, or Private JSON into Server-Side Web Tools

Understand the hidden data security risks of online utilities. Learn how server logs, session replay scripts, and third-party trackers expose credentials, and why client-side execution matters.

Read guide

More Developer Tools

View all →

JSON Formatter

Beautify, validate, minify, sort keys, and inspect JSON with interactive tree view.

Developer ToolsClient-Side

Base64 Encoder / Decoder

Encode and decode Base64 with URL-safe format, Data URIs, and live image preview.

Developer ToolsClient-Side