Regex Tester Guide

What is Regex Tester?

Our Regex Tester is a powerful tool for testing, debugging, and understanding regular expressions. It provides real-time pattern matching, automatic explanations, text substitution, code generation for 7 programming languages, and the ability to export matches in multiple formats. Whether you're validating email addresses, extracting data, or building complex search patterns, our tool makes regex development fast and intuitive.

Key Features

🎯 Three Powerful Modes

Switch between different modes to match your workflow:

• Match Mode: Find and highlight all matches in your text with detailed match information
• Substitute Mode: Find and replace text with support for capture group references ($1, $2, etc.)
• Code Generator: Generate ready-to-use regex code for JavaScript, Python, PHP, Java, C#, Go, or Ruby

💡 Auto Pattern Explanation

As you type your regex pattern, the tool automatically generates plain-English explanations of what each part does:

• Understand anchors like ^ (start) and $ (end)
• Learn about character classes (\d, \w, \s)
• See how quantifiers (*, +, ?, {n,m}) work
• Comprehend groups and alternation

🚩 Interactive Flags

Toggle regex flags with visual buttons to control pattern behavior:

• g - Global: Find all matches instead of just the first
• i - Case Insensitive: Match regardless of letter case
• m - Multiline: ^ and $ match line starts/ends instead of string starts/ends
• s - Dotall: Dot (.) matches newline characters

⚡ 12 Common Pattern Library

Quick-select from 12 pre-built patterns for common use cases:

• Email: Validate email addresses
• URL: Match web addresses
• Phone (US): Match US phone numbers in various formats
• IP Address: Match IPv4 addresses
• Hex Color: Match #RGB and #RRGGBB color codes
• Date (YYYY-MM-DD): Match ISO date format
• Credit Card: Match credit card numbers
• Username: Validate usernames (3-16 characters)
• Password (Strong): Enforce strong password requirements
• Hashtag: Match social media hashtags
• HTML Tag: Match HTML tags
• Numbers: Match integers

📊 Enhanced Match Information

View detailed information about each match:

• Match number badges for easy reference
• Exact character position in the text
• Length of each match
• Capture groups labeled as $1, $2, $3, etc.
• Yellow highlighting in the original text
• Scrollable list for handling many matches

🔄 Substitution Mode

Replace matched text with custom replacements:

• Use $1, $2, $3 to reference capture groups
• See real-time preview of substitution results
• Copy the substituted text with one click
• Perfect for reformatting data or cleaning text

💻 Multi-Language Code Generator

Generate production-ready code for 7 popular programming languages:

• JavaScript: With both match() and matchAll() examples
• Python: Using the re module with proper flags
• PHP: With preg_match and preg_match_all
• Java: Using Pattern and Matcher classes
• C#: With Regex class and RegexOptions
• Go: Using the regexp package
• Ruby: With scan and match methods

📤 Export Matches

Export your regex results in three different formats:

• Text: Simple text file with match details, positions, and lengths
• JSON: Structured data with match objects, groups, and metadata
• CSV: Spreadsheet-compatible format for data analysis

📖 Comprehensive Quick Reference

Access a complete regex cheat sheet organized into 6 categories:

• Character Classes: \d, \D, \w, \W, \s, \S, ., [abc], [^abc], [a-z]
• Quantifiers: *, +, ?, {n}, {n,}, {n,m}, *?, +?
• Anchors & Boundaries: ^, $, \b, \B, \A, \Z
• Groups & Lookaround: (abc), (?:abc), (?=abc), (?!abc), (?<=abc), (?<!abc), \1
• Special Characters: |, \, \n, \r, \t, \0
• Flags: g, i, m, s, u, y

How to Use the Regex Tester

Step 1: Choose Your Mode

Select one of three modes based on what you want to do:

• Match Mode: For finding and analyzing patterns in text
• Substitute Mode: For find-and-replace operations
• Code Generator: For generating code in your preferred language

Step 2: Enter Your Pattern

Type or select a regex pattern. You can:

• Type your own pattern from scratch
• Click a quick pattern button to start with a common pattern
• Modify an existing pattern to fit your needs

Step 3: Configure Flags

Click flag buttons to enable/disable regex modifiers:

• Blue = enabled, Gray = disabled
• Hover over flags to see tooltips explaining what they do
• Most patterns work best with the 'g' (global) flag enabled

Step 4: Enter Test String

Paste or type the text you want to test your pattern against. The tool supports:

• Single lines or multiple paragraphs
• Special characters and Unicode
• Large amounts of text (tested up to 100KB+)

Step 5: View Results

Analyze your results in multiple views:

• Match Count: See how many matches were found
• Highlighted Text: Matches are highlighted in yellow
• Match Information: Detailed cards for each match with position and groups
• Pattern Explanation: Plain-English breakdown of your regex

Step 6: Export or Copy

Use the action buttons to:

• Copy Matches: Copy all matched text to clipboard
• Export: Download as Text, JSON, or CSV
• Copy Code: (Code Generator mode) Copy generated code
• Copy Result: (Substitute mode) Copy the replaced text

Common Regex Patterns Explained

Email Validation

[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}

Matches standard email addresses. Allows letters, numbers, and common email characters before the @, followed by a domain name with at least a 2-letter extension.

US Phone Number

\(?\d{3}\)?[\s.-]?\d{3}[\s.-]?\d{4}

Matches US phone numbers in formats like: (123) 456-7890, 123-456-7890, 123.456.7890, or 1234567890.

URL Matching

https?://[\w\-]+(\.[\w\-]+)+[/#?]?.*

Matches HTTP and HTTPS URLs with domain names, optional paths, query strings, and fragments.

IPv4 Address

\b(?:\d{1,3}\.){3}\d{1,3}\b

Matches IPv4 addresses like 192.168.1.1 or 10.0.0.255. Note: This doesn't validate that numbers are 0-255.

Hex Color Code

#[0-9A-Fa-f]{6}\b|#[0-9A-Fa-f]{3}\b

Matches both 6-digit (#FF5733) and 3-digit (#F73) hex color codes.

ISO Date Format

\d{4}-\d{2}-\d{2}

Matches dates in YYYY-MM-DD format like 2024-01-15.

Advanced Techniques

Capture Groups

Use parentheses to capture parts of your match for later reference:

(\d{3})-(\d{3})-(\d{4})

This pattern captures three groups: area code ($1), prefix ($2), and line number ($3). In Substitute mode, you can rearrange them like: ($1) $2-$3

Non-Capturing Groups

Use (?:...) when you need grouping but don't need to capture:

(?:https?|ftp)://[\w.-]+

This matches URLs with http, https, or ftp protocols without creating a capture group.

Lookahead and Lookbehind

Match patterns only if they're followed or preceded by another pattern:

\$(?=\d+\.\d{2})  # Positive lookahead - $ before price
(?<=@)\w+          # Positive lookbehind - username after @

Lazy Quantifiers

Add ? after quantifiers to match as few characters as possible:

<.*?>    # Lazy - matches shortest tag
<.*>     # Greedy - matches longest span including multiple tags

Practical Use Cases

Validating User Input

Test form validation patterns before implementing them in your application:

• Email addresses with the Email pattern
• Strong passwords with complexity requirements
• Usernames with character and length restrictions
• Phone numbers in various formats

Data Extraction

Extract specific information from larger text documents:

• Pull email addresses from text
• Extract dates from logs
• Find all URLs in documents
• Parse structured data formats

Text Reformatting

Use Substitute mode to reformat data:

• Change date formats from MM/DD/YYYY to YYYY-MM-DD
• Reformat phone numbers to a standard format
• Add or remove parentheses around area codes
• Convert between different naming conventions

Log File Analysis

Parse and extract information from log files:

• Extract error messages and timestamps
• Find IP addresses in access logs
• Parse HTTP status codes
• Match specific log levels (ERROR, WARN, INFO)

Code Generation

Develop regex patterns and immediately generate code:

• Test pattern in the browser
• Switch to Code Generator mode
• Select your programming language
• Copy production-ready code with proper syntax and flags

Tips for Writing Better Regex

• Start Simple: Begin with basic patterns and add complexity gradually
• Test Edge Cases: Try your pattern with unusual but valid inputs
• Use Comments: In complex patterns, use the (?#comment) syntax to document your intent
• Escape Special Characters: Remember to escape characters like ., *, +, ?, [, ], {}, (, ), ^, $, |, \
• Optimize for Performance: Avoid excessive backtracking with lazy quantifiers and atomic groups
• Consider Readability: Use non-capturing groups and named groups for clarity
• Test with Real Data: Always test with actual data from your use case
• Use Anchors: Add ^ and $ to ensure the entire string matches, not just a substring
• Leverage Quick Patterns: Start with our pre-built patterns and modify them
• Check Explanations: Use the auto-generated explanations to verify your understanding

Common Regex Mistakes to Avoid

• Forgetting to Escape: Special characters like . and * must be escaped (\. and \*) to match literally
• Greedy Quantifiers: Using .* can match more than intended; use .*? for lazy matching
• Missing Global Flag: Without 'g', only the first match is found
• Case Sensitivity: Remember to use the 'i' flag for case-insensitive matching
• Backslash Confusion: In strings, you may need double backslashes (\\d instead of \d)
• Over-Complicating: Simple patterns are often better than complex ones
• Not Testing Thoroughly: Always test with multiple inputs, including edge cases
• Ignoring Performance: Complex regex can be slow on large texts

Frequently Asked Questions

How do I match a literal dot or asterisk?

Escape special characters with a backslash: \. matches a literal dot, \* matches a literal asterisk. Our pattern explanation feature will help you identify which characters need escaping.

What's the difference between greedy and lazy quantifiers?

Greedy quantifiers (*, +, {n,m}) match as much as possible. Lazy quantifiers (*?, +?, {n,m}?) match as little as possible. For example, <.*> in "<a><b>" matches the entire string, while <.*?> matches just "<a>".

Can I use regex for HTML parsing?

While you can match simple HTML tags with regex, it's not recommended for complex HTML parsing. HTML is not a regular language, and regex can't handle nested tags properly. Use a proper HTML parser for production code.

How do I test multiline text?

Enable the 'm' (multiline) flag to make ^ and $ match the start and end of lines instead of the entire string. Enable the 's' (dotall) flag to make . match newline characters.

What are capture groups used for?

Capture groups (using parentheses) let you extract specific parts of a match. In Match mode, you can see the captured groups for each match. In Substitute mode, you can reference them with $1, $2, etc. to rearrange or reuse parts of the match.

Can I save my regex patterns?

While the tool doesn't have a built-in save feature, you can use Code Generator mode to export your pattern in your preferred programming language, then save that code file for future reference.

Which programming language should I choose in Code Generator?

Choose the language you're using in your project. Each language has slightly different regex syntax and APIs. Our code generator handles these differences automatically and provides idiomatic code for each language.

Why doesn't my pattern match anything?

Common reasons include:

• Forgetting to escape special characters
• Using the wrong flags (enable 'g' for multiple matches, 'i' for case-insensitive)
• Pattern is too specific or too complex
• Check the Pattern Explanation section for insights

Can I export matches for use in Excel?

Yes! Use the Export dropdown and select CSV format. The CSV file can be opened in Excel, Google Sheets, or any spreadsheet application. It includes columns for Match, Position, and Length.

Is there a limit to how much text I can test?

The tool can handle large amounts of text (tested up to 100KB+), but very complex patterns on very large texts may slow down your browser. For best performance, test with representative samples first.