Regular Expression Tester

How to Use

1. Enter Pattern: Type your regular expression pattern in the pattern field
2. Set Flags: Select appropriate regex flags (g, i, m, s, u, y)
3. Enter Test Text: Add the text you want to test against your pattern
4. View Matches: See highlighted matches appear in real-time as you type
5. Review Details: Check captured groups, match positions, and match count
6. Refine Pattern: Adjust your regex based on the results until it works perfectly

Regex Flags Explained

g - Global

Find all matches in the text instead of stopping after the first match:

• Without g: Returns only the first match
• With g: Returns all matches in the text
• Essential for finding multiple occurrences
• Most commonly used flag

i - Case Insensitive

Ignore case when matching:

• Pattern "hello" matches "Hello", "HELLO", "HeLLo"
• Useful for user input validation
• Simplifies patterns that need to match any case
• Commonly combined with other flags

m - Multiline

Change behavior of ^ and $ anchors:

• ^ matches the start of each line (not just string start)
• $ matches the end of each line (not just string end)
• Essential for processing multi-line text
• Useful for line-by-line validation

s - DotAll

Make the dot (.) match newline characters:

• Normally . matches any character except newlines
• With s flag, . matches everything including
• Useful for matching across multiple lines
• Simplifies patterns that span lines

u - Unicode

Enable full Unicode support:

• Properly handle Unicode characters and emoji
• Enable Unicode property escapes like p{Letter}
• Correct handling of surrogate pairs
• Recommended for international text

y - Sticky

Match only from the lastIndex position:

• Matches must start at the exact lastIndex position
• Useful for parsing and tokenization
• More restrictive than global flag
• Advanced use case for sequential matching

Common Use Cases

Data Validation

• Email Validation: Test patterns for validating email addresses
• Phone Numbers: Validate phone number formats
• URLs: Check URL patterns and extract components
• Postal Codes: Validate zip codes and postal codes
• Credit Cards: Test credit card number patterns
• Dates: Validate date formats (MM/DD/YYYY, etc.)

Text Processing

• Extract Data: Pull specific information from text
• Find Patterns: Locate specific text patterns
• Search & Replace: Test patterns before replacing text
• Parse Logs: Extract information from log files
• Clean Data: Identify and remove unwanted characters

Development & Debugging

• Debug Regex: Test and fix regex patterns that aren't working
• Learn Regex: Experiment with patterns to understand behavior
• Optimize Patterns: Test different approaches for efficiency
• Validate Input: Test form validation patterns
• API Development: Test patterns for API parameter validation

Web Scraping

• Extract URLs: Find and extract links from HTML
• Parse HTML: Extract specific elements or attributes
• Find Patterns: Locate specific data patterns in web content
• Clean Text: Remove HTML tags or unwanted formatting

Common Regex Patterns

Basic Patterns

• Email: [a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+.[a-zA-Z]{2,}
• URL: https?://[^s]+
• Phone (US): d{3}-d{3}-d{4} or (d{3})s?d{3}-d{4}
• Date (MM/DD/YYYY): d{2}/d{2}/d{4}
• Time (HH:MM): d{2}:d{2}
• Hex Color: #[0-9A-Fa-f]{6}
• IP Address: d{1,3}.d{1,3}.d{1,3}.d{1,3}

Character Classes

• d: Any digit (0-9)
• w: Any word character (a-z, A-Z, 0-9, _)
• s: Any whitespace (space, tab, newline)
• D: Any non-digit
• W: Any non-word character
• S: Any non-whitespace
• .: Any character (except newline without s flag)

Quantifiers

• *: 0 or more times
• +: 1 or more times
• ?: 0 or 1 time (optional)
• {n}: Exactly n times
• {n,}: n or more times
• {n,m}: Between n and m times

Anchors & Boundaries

• ^: Start of string (or line with m flag)
• $: End of string (or line with m flag)
• : Word boundary
• B: Non-word boundary

Understanding Match Results

Match Highlighting

Matches are highlighted in the test text:

• Each match is visually highlighted
• Multiple matches shown with global flag
• Overlapping matches handled appropriately
• Easy visual confirmation of pattern behavior

Capture Groups

View captured groups from your pattern:

• Parentheses () create capture groups
• Each group's content is displayed
• Numbered groups (1, 2, 3, etc.)
• Useful for extracting specific parts

Match Information

Detailed information about each match:

• Total number of matches found
• Position of each match in the text
• Length of each match
• Captured group values

Best Practices

Pattern Development

• Start simple and add complexity gradually
• Test with multiple examples including edge cases
• Use the global flag to see all matches
• Check both positive and negative test cases
• Consider performance for large texts

Common Mistakes to Avoid

• Forgetting to escape special characters (. * + ? etc.)
• Not using the global flag when finding all matches
• Overly greedy quantifiers (use ? for non-greedy)
• Not testing with edge cases and invalid input
• Making patterns too complex when simple works

Optimization Tips

• Use specific character classes instead of . when possible
• Anchor patterns with ^ and $ when appropriate
• Use non-capturing groups (?:) when you don't need the capture
• Avoid excessive backtracking with careful quantifier use
• Test performance with realistic data sizes

Key Features

• Real-Time Testing: See matches instantly as you type
• All JavaScript Flags: Support for g, i, m, s, u, y flags
• Match Highlighting: Visual highlighting of all matches
• Capture Groups: View captured group values
• Match Details: Position and count information
• Error Handling: Clear error messages for invalid patterns
• No Limits: Test patterns on any size text
• Complete Privacy: All processing happens locally

Tips & Tricks

• Use the global (g) flag to see all matches, not just the first one
• Test your pattern with various inputs including edge cases
• Start with simple patterns and add complexity incrementally
• Use capture groups () to extract specific parts of matches
• Enable case-insensitive (i) flag for flexible text matching
• Use multiline (m) flag when working with multi-line text
• Check the match count to verify you're finding all expected matches
• Test both valid and invalid inputs to ensure pattern accuracy
• Use  for word boundaries to match whole words only
• Remember to escape special regex characters with backslash

Common Scenarios

Scenario 1: Email Validation

You need to validate email addresses in a form. Test the pattern [a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+.[a-zA-Z]{2,} with various email formats to ensure it catches valid emails and rejects invalid ones.

Scenario 2: Extract Phone Numbers

You have text containing phone numbers in various formats. Use the pattern (?d{3})?[-.s]?d{3}[-.s]?d{4} with the global flag to find all phone numbers regardless of formatting.

Scenario 3: Parse Log Files

You need to extract timestamps from log files. Test a pattern like d{4}-d{2}-d{2}sd{2}:d{2}:d{2} to match ISO format timestamps and verify it captures all log entries.

Scenario 4: Clean HTML Tags

You want to remove HTML tags from text. Test the pattern <[^>]+> with the global flag to match all HTML tags and verify it doesn't accidentally match legitimate < or > characters in content.