Regex Tester

Test regex with support for JavaScript, Python, PHP & Java. 75+ templates, match highlighting, capture groups & /pattern/flags syntax. Free online regex tester.

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Frequently Asked Questions

Quick answers to common questions

What is a regex tester and how does it help developers?

A regex tester is an interactive tool that allows you to test and debug regular expressions in real-time. It dramatically speeds up pattern development by: showing matches instantly as you type with color-coded highlighting, displaying capture groups and their contents, explaining what each part of your pattern does, supporting multiple regex flavors (JavaScript, Python, PHP, Java), providing error messages for invalid syntax, offering pre-built templates for common patterns. Our tool uses the browser's built-in RegExp engine for accurate, language-specific matching. This means when you test in JavaScript mode, you're seeing exactly what JavaScript would match—no cross-language surprises. The live feedback loop eliminates the tedious write-compile-test cycle, letting you iterate on patterns in seconds rather than minutes.

What programming languages and regex flavors are supported?

Our regex tester supports four major regex engines with their unique syntax differences: JavaScript (ES2024+)—uses /pattern/flags syntax, supports lookahead/lookbehind, Unicode property escapes, named capture groups. Python (re module 3.11+)—supports most PCRE features, verbose mode with comments, conditional patterns, possessive quantifiers. PHP (PCRE2)—full Perl-compatible feature set, recursive patterns, subroutines, callouts, backtracking control verbs. Java (java.util.regex)—supports Unicode categories, canonical equivalence, grapheme clusters, possessive quantifiers. Switch between languages using the dropdown—the interface updates to show language-specific features. Important: syntax differences matter—\w in JavaScript matches [A-Za-z0-9_] while in Python 3 it matches Unicode word characters including accented letters. Always test in the language you'll use.

How do I use flags and modifiers in my regex patterns?

Regex flags change how the engine interprets your pattern. Available flags vary by language: g (global)—find all matches, not just the first (JavaScript, PHP, Java). i (case-insensitive)—match uppercase and lowercase equally. m (multiline)—makes ^ and $ match line beginnings/endings, not just string boundaries. s (dotAll/single-line)—makes . match newlines. u (Unicode)—full Unicode support in JavaScript. x (verbose)—allows whitespace and comments in Python/PHP. Our tool provides checkboxes for common flags—no need to remember syntax like /pattern/gim. Click flags to toggle them instantly and see results update. Examples: Use i for case-insensitive email matching, m for matching line-by-line in log files, s for matching across multiple lines, g to find all occurrences in a text. Some languages combine flags: /pattern/gim in JavaScript, pattern(?i) in PHP inline flags. Pro tip: start with flags that match your use case, then adjust if you're matching too much or too little.

What are capture groups and how do I use them?

Capture groups let you extract specific parts of matches using parentheses (). They're essential for data extraction and transformation. Basic capture group: (\d{3}) captures three digits. Access as $1 or group[1] in replacement. Named capture group: (?\d{4}) captures four digits and names it 'year'. Access as ${year} or match.groups.year. Non-capturing group: (?:\d{3}) groups without capturing—faster when you don't need the value. Our tool shows all capture groups in a dedicated panel: group number, name (if named), captured value, start/end positions. Use cases: extracting area codes from phone numbers, parsing dates into year/month/day components, validating and restructuring URLs, cleaning up data formats. Example: (?\d{3})-(?\d{3})-(?\d{4}) captures US phone number components. Access ${area}, ${exchange}, ${number} in replacement text. Best practice: use named groups for complex patterns—they're self-documenting and easier to maintain.

What are regex templates and when should I use them?

Regex templates are pre-built, tested patterns for common validation and extraction tasks. They save time and prevent reinventing well-tested solutions. Our templates include: Email validation—RFC-compliant pattern that handles edge cases, URL matching—parses protocol, domain, path, query strings, IP address validation—both IPv4 and IPv6 patterns, Phone number extraction—flexible patterns for multiple formats, Date/time parsing—ISO 8111, US, international formats, Credit card validation—Luhn algorithm with format checking, HTML tag extraction—balanced tag matching, Password strength—complexity requirements. Click any template to load it instantly, then customize for your specific needs. Templates are starting points, not rigid solutions—you might need to adjust them for: specific country formats (phone numbers, postal codes), business rules (password policies, username constraints), legacy data patterns. Each template includes comments explaining what it matches and why. Pro tip: use templates as learning tools—study them to understand advanced regex techniques like lookarounds, possessive quantifiers, and atomic groups.

Can I save, export, and share my regex patterns?

Yes! Our tool provides multiple ways to save and share your work: Local history—patterns automatically save to browser's local storage as you work, accessible from the history panel on the left, persists across sessions, no account required. Copy to clipboard—click to copy pattern with flags in various formats: JavaScript /pattern/flags, Python raw strings r'pattern', PHP /pattern/flags, Java "pattern". Share via URL—patterns encode in the URL hash, send links to colleagues for collaboration, bookmark patterns for quick access. Export options—copy as code snippet ready to paste into your project, download as JSON for backup and version control. Privacy note: since everything runs locally, your patterns never leave your browser unless you explicitly share them. This makes our tool safe for proprietary business logic, sensitive data patterns, internal validation rules. Pro workflow: develop and test patterns in our tool, then copy the final version into your codebase with confidence.

How do I debug complex regex patterns that aren't working as expected?

Debugging regex requires systematic analysis. Our tool provides several debugging features: Match explanation—hover over any part of your pattern to see what it does, color-coded syntax highlighting groups related elements together. Error detection—invalid syntax shows red underlines with specific error messages, helps you catch unclosed groups, invalid escapes, misplaced quantifiers. Test cases panel—save multiple test strings with expected results, quickly verify edge cases, ensure your pattern handles all scenarios. Step-by-step matching (coming soon)—watch the engine match character by character, see backtracking in action, understand why matches fail or succeed. Debugging strategy: 1) Start simple—test with minimal input, verify basic matching works, 2) Add complexity gradually—build up pattern piece by piece, 3) Use test cases—try edge cases like empty strings, special characters, boundary values, 4) Check for overmatching—ensure you're not matching too broadly, 5) Test negative cases—confirm what shouldn't match doesn't, 6) Read the error messages carefully—they often point to the exact problem. Common gotchas: forgetting to escape special characters (\. vs .), using * when you mean + (zero vs one or more), mixing up ^ and $ anchors in multiline mode, relying on implicit word boundaries in different languages.

What are lookaheads, lookbehinds, and other advanced regex features?

Advanced regex features enable sophisticated matching beyond simple patterns: Lookahead (?=pattern)—matches if pattern follows, without consuming it. Example: \w+(?=ing) matches words ending in 'ing' but excludes 'ing' from the match. Negative lookahead (?!pattern)—matches if pattern doesn't follow. Example: \d+(?!\.\d) matches integers not followed by decimals. Lookbehind (?<=pattern)—matches if pattern precedes (limited width in some engines). Example: (?<=\$)\d+ matches numbers after $ sign. Negative lookbehind (?group)—prevent backtracking for performance. Example: (?>\d+) fails immediately if no digits follow, doesn't backtrack. Possessive quantifiers ++, *+, ?+—like atomic groups, prevent backtracking. Backreferences \1, \g—match previously captured text. Example: (\w+)\s+\1 matches repeated words. Conditional patterns (?(if)then|else)—branch based on whether a group captured. Our tool's syntax reference explains these features with examples. Use cases: validating password requirements without consuming characters, matching quoted strings with escaped quotes, parsing balanced pairs, implementing business rules in validation. Warning: lookarounds can cause performance issues with complex patterns or large inputs. Use them judiciously and test with realistic data volumes. Always consider whether a simpler approach (multiple regexes, string parsing) might be more maintainable.

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Use Cases

Discover how to integrate this tool into your workflow

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Text Processing & String Manipulation

Test and validate regular expressions for search, extraction, and transformation of text data.

Pattern matching for text search with flexible criteria
Extract specific data like dates, codes, or identifiers
Validate string formats against business rules
Search and replace with capture groups for restructuring
Parse CSV or fixed-width text files programmatically
Clean and normalize user input from web forms

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Form Validation & Data Cleaning

Create regex patterns for client-side and server-side validation of user input data.

Email address validation with RFC-compliant patterns
Phone number formatting for international formats
URL parsing and validation for security
Input sanitization to prevent XSS and injection attacks
Password strength validation with complexity requirements
Postal code and address format validation

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Log Analysis & Debugging

Extract specific patterns, errors, and metrics from server logs and application traces.

Match error patterns and stack traces in application logs
Filter logs by severity level or error codes
Extract IP addresses, timestamps, and user IDs
Parse user agent strings to identify browsers and devices
Extract performance metrics from monitoring logs
Correlate related log entries across distributed systems

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Web Scraping & Data Extraction

Build regex patterns to extract structured data from HTML, XML, and text documents.

Extract links and image URLs from web pages
Parse HTML tags and attributes (with caution)
Scrape product prices and descriptions from e-commerce sites
Extract metadata from document headers
Parse JSON-like structures from text responses
Clean and normalize scraped data before storage

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Code Refactoring & Migration

Use regex to find and transform code patterns during refactoring projects and language migrations.

Rename variables or functions across multiple files
Update import statements and module paths
Convert between coding conventions (camelCase to snake_case)
Replace deprecated API calls with new versions
Extract magic numbers into named constants
Standardize string quote styles and indentation

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Security & Compliance

Detect sensitive information patterns and enforce security policies in text data.

Detect credit card numbers, SSNs, and PII in logs
Find hardcoded passwords and API keys in source code
Validate input against security patterns and allowlists
Scan for SQL injection and XSS attack patterns
Enforce password policies with complexity requirements
Identify potential security vulnerabilities in configuration files

About This Tool

Regular expressions are powerful but a pain to write. This tool lets you test your regex against sample text in real-time—highlight matches, see capture groups, debug why your pattern isn't working. Supports all the modern JS regex features (lookbehind, named groups, Unicode stuff) that most online testers don't handle. Saved me hours of trial-and-error debugging complex patterns.

Technical Details

Uses JavaScript's RegExp with ES2018+ features. You've got your standard flags (g, i, m, s, u, y) and the newer lookbehind assertions, named capture groups, Unicode property escapes. The match highlighting shows exact positions, capture groups are displayed separately, and you get helpful error messages if your syntax is wrong (instead of just silently failing like some tools).

Algorithm

Under the hood it's the standard ECMAScript regex engine—backtracking NFA algorithm if you're curious. Finds all matches with the global flag, or just the first one without it. Real-time matching updates as you type, which is way better than having to hit "test" after every change. Nothing proprietary, just solid use of browser APIs.

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Privacy Commitment

🔒 **Privacy First**: Unlike server-based tools, AI-TOL processes everything locally in your browser - your data never leaves your device. No uploads, no tracking, completely private.