FreeKit is a collection of free online tools for developers, SEO professionals, designers, students, content creators, and everyday users.

Are all tools completely free?

Yes. Every tool on FreeKit is free to use without subscriptions or hidden charges.

Do I need to create an account?

No. You can use all tools instantly without creating an account.

No. Most tools process data locally in your browser, and we do not permanently store your inputs.

Can I use FreeKit on mobile devices?

Yes. Every tool is fully responsive and works on smartphones, tablets, laptops, and desktop computers.

How often are new tools added?

We regularly add new tools and improve existing ones based on user feedback and industry needs.

Which browsers are supported?

FreeKit supports all modern browsers, including Chrome, Edge, Firefox, Safari, and other Chromium-based browsers.

Can I share these tools with others?

Absolutely. You are welcome to share FreeKit with friends, colleagues, students, and your team.

What is a regular expression (regex)?

A regular expression (regex or regexp) is a sequence of characters that defines a search pattern. It is used in programming and text processing to find, match, validate, or replace specific text within a larger string. For example, the regex `\d+` matches one or more digits, and `[a-z]+` matches one or more lowercase letters. Regex is supported by virtually every programming language and is essential for data validation, text parsing, and search operations.

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

Each flag modifies how the regex engine interprets your pattern. The `g` (global) flag finds all matches in the string instead of stopping at the first one. The `i` (case-insensitive) flag makes the match ignore letter case, so `/abc/i` matches 'ABC', 'abc', and 'Abc'. The `m` (multiline) flag makes `^` and `$` match the start and end of each line rather than just the entire string. The `s` (dotall) flag makes `.` match newline characters too. The `u` (unicode) flag enables full Unicode support for matching emoji, accented characters, and other Unicode symbols.

What are capture groups and how do I use them?

Capture groups are portions of your regex enclosed in parentheses `()` that 'remember' the matched text for later use. For example, in the pattern `(\w+)@(\w+)\.(\w+)`, there are three capture groups: the username, the domain name, and the top-level domain. After a match, you can reference these groups by their index (group 1, group 2, etc.) or by named groups using `(? ...)` syntax. In replacement strings, use `$1`, `$2`, etc. to insert captured values. Non-capturing groups `(?:...)` group without capturing.

What is the difference between lookahead and lookbehind assertions?

Lookahead and lookbehind are zero-width assertions that check for a pattern without including it in the match. A positive lookahead `(?=...)` asserts that what follows the current position matches the pattern. For example, `\d+(?=px)` matches digits only if followed by 'px' (like '16' in '16px'). A negative lookahead `(?!...)` asserts the opposite. A positive lookbehind `(?<=...)` asserts that what precedes the current position matches, like `(?<=\$)\d+` to match dollar amounts. A negative lookbehind `(?<!...)` ensures the pattern is NOT preceded by the given text. These are powerful for context-aware matching.

Why does my regex pattern cause an error?

Regex syntax errors occur when the pattern contains invalid constructs. Common causes include: unescaped forward slashes (use `\/` to match a literal `/`), unbalanced parentheses or brackets, invalid escape sequences (like `\x` without two hex digits), invalid quantifiers (like `*+` or `?{`), and incompatible flag combinations. Our tool displays the exact error message from the JavaScript regex engine so you can identify and fix the issue. If you are migrating regex from another language (Python, Java, PCRE), note that JavaScript regex has some syntax differences.

How do I test a regex for email validation?

Email validation with regex is a classic challenge because the official specification (RFC 5322) is extremely complex. For practical use, a simplified pattern like `^[\w.-]+@[\w.-]+\.[a-zA-Z]{2,}$` covers most common email formats. You can click the 'Email' quick-insert button in our tool to load this pattern and test it against various email addresses. For production use, combine regex validation with server-side verification (sending a confirmation email) for the most reliable results.

Can I use this tool to test regex from Python, Java, or other languages?

Our tool uses JavaScript's built-in RegExp engine, which follows the ECMAScript specification. While most regex syntax is shared across languages, there are differences. For example, Python supports `\A` and `\Z` for absolute start/end, Java uses `\p{L}` for Unicode categories, and .NET supports balancing groups. The core patterns like character classes, quantifiers, groups, and lookarounds work the same everywhere. Test your patterns here first, then verify any language-specific features in your target environment.

What is the difference between greedy and lazy (non-greedy) quantifiers?

By default, quantifiers like `*`, `+`, and `{n,m}` are greedy, meaning they match as much text as possible. For example, ` ` matches everything from the first ` ` in a string. Adding a `?` after a quantifier makes it lazy (non-greedy), so it matches as little as possible. ` ` matches from ` `. Understanding greedy vs. lazy behavior is crucial for writing correct regex, especially when parsing HTML, XML, or other structured text with repeating delimiters.

How does the replacement preview feature work?

The replacement feature lets you specify a replacement string and see the result in real time. In the replacement string, you can use `$&` to refer to the entire match, `$1`, `$2`, etc. to refer to numbered capture groups, `$ ` for named capture groups, and `$$` for a literal dollar sign. For example, with the pattern `(\d{4})-(\d{2})-(\d{2})` and replacement `$2/$3/$1`, the date '2025-07-22' becomes '07/22/2025'. The preview updates live as you type either the pattern or the replacement string.

Is my test data secure when using this regex tester?

Yes, completely. All regex testing, matching, and replacement operations run entirely in your browser using client-side JavaScript. Your test strings, regex patterns, and any other data you enter are never sent to any server, stored in a database, or transmitted over the network. You can safely test regex patterns against sensitive data like API responses, log entries, personal information, or confidential text without any privacy concerns.

Regex Tester

Test and debug regular expressions with real-time matching and highlighting.

IntermediateDeveloper ToolsFreev1.0.0Updated Jul 22, 2025

Regex Pattern

//g

Flags

Common Patterns — click to insert

Test String0 chars

Match Highlighting

Highlighted matches will appear here...

Replacement String

100% Private — Your Data Never Leaves Your Browser

All regex testing, matching, and replacement operations run entirely in your browser. Your patterns and test data are never sent to any server.

How to Use the Regex Tester

Enter your regex pattern — Type or paste a regular expression pattern in the input field. You can also click one of the quick-insert buttons (Email, URL, Phone, IPv4, Date, Hex Color) to load a common pattern instantly.
Select your flags — Toggle the regex flags you need: g for global (all matches), i for case-insensitive, m for multiline, s for dotall, and u for Unicode.
Enter your test string — Paste or type the text you want to test the regex against. Matches are highlighted in real time as you type both the pattern and the test string.
Review match details — The Match Details panel shows each match with its index position, length, and matched value. If your pattern uses capture groups, the Capture Groups panel displays each group's value for every match.
Preview replacements — Enter a replacement string using $1, $2, $&, or $$ tokens to see a live preview of what the text looks like after replacement. This is invaluable for building substitution logic.

Features

Real-time match highlighting with color-coded markers in the test string
Detailed match panel showing index, length, and value for every match found
Capture groups display with support for both numbered and named groups
Live replacement preview using $1, $2, $&, $$, and $<name> tokens
Five regex flags with toggle buttons: global, case-insensitive, multiline, dotall, Unicode
Six common pattern quick-insert buttons: Email, URL, Phone, IPv4, Date, Hex Color
Collapsible regex cheat sheet covering character classes, quantifiers, anchors, groups, lookarounds, and special characters
Instant error display for invalid regex patterns with the exact JavaScript error message
Copy Regex button copies the full pattern with flags (e.g., /d+/gi) to clipboard
100% client-side processing — your data never leaves your browser

Understanding Regular Expressions

What Are Regular Expressions?

Regular expressions (regex) are a powerful text-matching language used to search, validate, extract, and transform strings. Built from a combination of literal characters and special metacharacters, regex patterns can describe everything from simple word matches to complex multi-part validation rules. They are supported by every major programming language, most text editors, command-line tools like grep and sed, and database query engines.

Greedy vs. Lazy Matching

Quantifiers like *, +, and {n,m} are greedy by default, meaning they match as much text as possible. For example, <.*> matches from the first < to the last > in a string. Adding a ? after a quantifier makes it lazy (non-greedy), matching as little as possible. <.*?> matches from < to the very next >. Understanding this distinction is critical for correctly parsing HTML, log files, and structured text with repeating delimiters.

Capture Groups and Backreferences

Parentheses in a regex create capture groups that remember the matched text. Groups are numbered from left to right: (a)(b(c)) has three groups. Named groups (?<name>...) make patterns more readable. In replacement strings, $1, $2, etc. refer to captured values, and $& refers to the entire match. Non-capturing groups (?:...) group without capturing, improving performance when you only need grouping for alternation or quantifiers.

Lookahead and Lookbehind Assertions

Lookahead (?=...) and lookbehind (?<=...) are zero-width assertions that check for patterns without including them in the match. Positive lookahead asserts what follows; negative lookahead (?!...) asserts what does NOT follow. Lookbehind works similarly for what precedes the current position. These are essential for context-aware matching like finding words not followed by specific suffixes, or numbers preceded by currency symbols.

Common Use Cases

Form Input Validation

Build and test regex patterns for email, phone number, ZIP code, credit card, username, and other form field validation before implementing them in your application's frontend or backend.

Log File Analysis

Extract timestamps, IP addresses, error codes, request paths, and other structured data from server logs, application logs, and audit trails using targeted regex patterns.

Data Extraction and Scraping

Parse HTML, XML, CSV, or plain text to extract specific data fields like product prices, dates, URLs, or user information using capture groups and lookarounds.

Search and Replace in Code

Test bulk find-and-replace patterns for code refactoring, renaming variables across files, updating import paths, or migrating API endpoints.

URL and Path Routing

Design and test regex patterns for URL routing in web frameworks, REST API path matching, and query parameter extraction.

Text Cleanup and Normalization

Build patterns to remove extra whitespace, fix inconsistent capitalization, strip HTML tags, normalize date formats, or clean up imported data before processing.

Password Policy Testing

Verify regex patterns that enforce password complexity requirements like minimum length, required character types (uppercase, lowercase, digits, special characters), and forbidden patterns.

Learning and Teaching Regex

Students and educators can use the real-time highlighting, match details, and cheat sheet to learn regex concepts interactively. Experiment with patterns and immediately see what they match and why.

Regex Tester Tips

After testing a regex pattern, use our Text Replacer to perform bulk find-and-replace operations across large blocks of text using the patterns you have validated.
If you are validating JSON data and need to extract specific fields using regex, use our JSON Validator to first ensure your JSON is valid before applying patterns to it.
When you need to encode or decode strings that contain regex-special characters, use our Base64 Encoder to safely transmit them without worrying about escaping.
If your test string contains HTML entities and you want to test regex against the decoded text, use our HTML Entity Encoder to decode entities first, then paste the result here.
When testing patterns for validation (like email or phone), always test both valid and invalid inputs. The quick-insert buttons provide common patterns, but you should verify edge cases like empty strings, missing parts, and malformed formats.
Use the replacement preview feature to validate your substitution logic before applying it in code. Special tokens like $1, $2, $& (entire match), and $$ (literal dollar) are all supported and previewed in real time.
Toggle the 'm' (multiline) flag when your test string contains multiple lines and you want ^ and $ to match line starts/ends instead of the entire string boundary.
The cheat sheet below the tool provides a quick reference for character classes, quantifiers, anchors, groups, lookarounds, and special characters. Keep it open while building complex patterns.

Frequently Asked Questions

Related Tools

Text Replacer

Find and replace text with regex support and batch processing.

IntermediateFree

JSON Validator

Validate JSON data and get detailed error messages with line numbers.

BeginnerFree

Base64 Encoder/Decoder

Encode and decode Base64 strings and files.

BeginnerFree

HTML Entity Encoder

Encode and decode HTML entities for safe web content.

BeginnerFree

JSON Formatter

Format, validate, and beautify JSON data instantly.

BeginnerFree

UUID Generator

Generate random UUIDs (v4) instantly.

BeginnerFree

URL Encoder/Decoder

Encode and decode URLs and query parameters.

BeginnerFree

Faisal Hossain

Founder & Developer of FreeKit

I personally designed and built every tool on this website — including the one you just used. If these tools have been helpful, I can build custom tools, automation workflows, websites, and SEO solutions tailored to your business.

Services

SEO Audit, Technical SEO & On-Page SEO
WordPress & Shopify Development
WooCommerce & E-commerce Management
n8n & AI Workflow Automation
Python Automation & Web Scraping
VPS, Linux & Self-Hosted Infrastructure
AI Vibe Coding Expert — APPS, WEB, SAAS

Availability

Remote: Available Now

United States & European company

USD $25/hour or USD $1,500/month

On-site: Open to Relocation

With work visa & travel support

Email Me WhatsApp LinkedIn

View full profile & services

How to Use the Regex Tester

Enter your regex pattern — Type or paste a regular expression pattern in the input field. You can also click one of the quick-insert buttons (Email, URL, Phone, IPv4, Date, Hex Color) to load a common pattern instantly.

Select your flags — Toggle the regex flags you need: g for global (all matches), i for case-insensitive, m for multiline, s for dotall, and u for Unicode.

Enter your test string — Paste or type the text you want to test the regex against. Matches are highlighted in real time as you type both the pattern and the test string.

Review match details — The Match Details panel shows each match with its index position, length, and matched value. If your pattern uses capture groups, the Capture Groups panel displays each group's value for every match.

Preview replacements — Enter a replacement string using $1, $2, $&, or $$ tokens to see a live preview of what the text looks like after replacement. This is invaluable for building substitution logic.

Features

Real-time match highlighting with color-coded markers in the test string

Detailed match panel showing index, length, and value for every match found

Capture groups display with support for both numbered and named groups

Live replacement preview using $1, $2, $&, $$, and $<name> tokens

Five regex flags with toggle buttons: global, case-insensitive, multiline, dotall, Unicode

Six common pattern quick-insert buttons: Email, URL, Phone, IPv4, Date, Hex Color

Collapsible regex cheat sheet covering character classes, quantifiers, anchors, groups, lookarounds, and special characters

Instant error display for invalid regex patterns with the exact JavaScript error message

Copy Regex button copies the full pattern with flags (e.g., /d+/gi) to clipboard

100% client-side processing — your data never leaves your browser

Understanding Regular Expressions

What Are Regular Expressions?

Greedy vs. Lazy Matching

Capture Groups and Backreferences

Lookahead and Lookbehind Assertions

Common Use Cases

Form Input Validation

Build and test regex patterns for email, phone number, ZIP code, credit card, username, and other form field validation before implementing them in your application's frontend or backend.

Log File Analysis

Extract timestamps, IP addresses, error codes, request paths, and other structured data from server logs, application logs, and audit trails using targeted regex patterns.

Data Extraction and Scraping

Parse HTML, XML, CSV, or plain text to extract specific data fields like product prices, dates, URLs, or user information using capture groups and lookarounds.

Search and Replace in Code

Test bulk find-and-replace patterns for code refactoring, renaming variables across files, updating import paths, or migrating API endpoints.

URL and Path Routing

Design and test regex patterns for URL routing in web frameworks, REST API path matching, and query parameter extraction.

Text Cleanup and Normalization

Build patterns to remove extra whitespace, fix inconsistent capitalization, strip HTML tags, normalize date formats, or clean up imported data before processing.

Password Policy Testing

Verify regex patterns that enforce password complexity requirements like minimum length, required character types (uppercase, lowercase, digits, special characters), and forbidden patterns.

Learning and Teaching Regex

Students and educators can use the real-time highlighting, match details, and cheat sheet to learn regex concepts interactively. Experiment with patterns and immediately see what they match and why.

Regex Tester Tips

After testing a regex pattern, use our Text Replacer to perform bulk find-and-replace operations across large blocks of text using the patterns you have validated.

If you are validating JSON data and need to extract specific fields using regex, use our JSON Validator to first ensure your JSON is valid before applying patterns to it.

When you need to encode or decode strings that contain regex-special characters, use our Base64 Encoder to safely transmit them without worrying about escaping.

If your test string contains HTML entities and you want to test regex against the decoded text, use our HTML Entity Encoder to decode entities first, then paste the result here.

When testing patterns for validation (like email or phone), always test both valid and invalid inputs. The quick-insert buttons provide common patterns, but you should verify edge cases like empty strings, missing parts, and malformed formats.

Use the replacement preview feature to validate your substitution logic before applying it in code. Special tokens like $1, $2, $& (entire match), and $$ (literal dollar) are all supported and previewed in real time.

Toggle the 'm' (multiline) flag when your test string contains multiple lines and you want ^ and $ to match line starts/ends instead of the entire string boundary.

The cheat sheet below the tool provides a quick reference for character classes, quantifiers, anchors, groups, lookarounds, and special characters. Keep it open while building complex patterns.

Frequently Asked Questions