DevTools360

Regex Tester

Test and debug regular expressions in real time

Regex Pattern
Test String
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Match Results
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Privacy First: All regex testing happens locally in your browser. Your data is never sent to a server or stored.

Regex Tester Online

Test regular expressions in real time with match highlighting, capture groups, and flag support. This regex tester helps you debug patterns safely with timeout protection.

This page is the canonical regex tester for DevTools360 and is updated regularly to match modern JavaScript regex behavior.

How to Use the Regex Tester

  1. Enter a pattern: Add your regex and flags.
  2. Add test text: Paste the string you want to match.
  3. Review matches: See highlights and capture groups.
  4. Iterate safely: Use the timeout guard to avoid freezes.

Example Regex Match

Pattern:

/\b[\w.-]+@[\w.-]+\.[A-Za-z]{2,}\b/g

Test text:

Contact: ada@example.com, grace@example.org

Common Regex Patterns and Use Cases

Here are some common regex patterns that developers use frequently:

  • Email validation: \b\w+@\w+\.\w+\b - Matches basic email addresses
  • Phone numbers: \d{3}-\d{3}-\d{4} - Matches XXX-XXX-XXXX format
  • URLs: https?://[^\s]+ - Matches HTTP/HTTPS URLs
  • IP addresses: \d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3} - Matches IPv4 addresses
  • Dates: \d{4}-\d{2}-\d{2} - Matches YYYY-MM-DD format
  • Credit cards: \d{4}[\s-]?\d{4}[\s-]?\d{4}[\s-]?\d{4} - Matches card numbers with optional separators
  • Hex colors: #[0-9A-Fa-f]{6} - Matches hexadecimal color codes

These patterns can be customized for your specific needs. For example, when working with JSON data that contains URLs, you can use our JSON Viewer tool to inspect the structure, then use regex patterns to extract specific values. Similarly, when processing URLs, our URL Encode tool can help encode special characters that might need escaping in regex patterns.

Common Regex Errors

  • Catastrophic backtracking: Avoid nested quantifiers like (a+)+.
  • Greedy matching: Use lazy quantifiers like .*? when needed.
  • Unescaped characters: Escape . + * ? ( ) [ ] in literals.
  • Wrong flags: Add g for global matches, m for multiline.

Understanding Regex Flags

Regex flags modify how a pattern behaves during matching. Our regex tester supports all standard JavaScript flags:

  • g (global): Find all matches instead of stopping after the first match. Essential when you need to find multiple occurrences in a string.
  • i (case insensitive): Makes the pattern match regardless of case. Useful for user input validation where case shouldn't matter.
  • m (multiline): Makes ^ and $ match the start and end of each line, not just the start and end of the entire string. Crucial for processing multi-line text.
  • s (dotall): Makes the . character match newline characters as well. Useful when you need to match across line breaks.

Flags can be combined. For example, gi enables both global matching and case-insensitive matching. Understanding flags is crucial for writing effective regex patterns that work correctly in different scenarios.

Performance Pitfalls and Best Practices

Regular expressions can be powerful but also dangerous if not used carefully. Here are common performance pitfalls and how to avoid them:

Catastrophic Backtracking

This occurs when a regex pattern has exponential time complexity, causing it to freeze or take extremely long to execute. Common causes include:

  • Nested quantifiers: (a+)+ or (.*)*
  • Alternations with overlapping patterns: (a|ab)*
  • Greedy quantifiers on patterns that can match empty strings

Solutions: Use atomic groups when possible, avoid nested quantifiers, prefer specific character classes over ., and test with realistic data sizes. Our regex tester includes timeout protection to prevent browser freezing.

Best Practices

  • Be specific: Use \d instead of [0-9] when possible
  • Avoid unnecessary capturing: Use non-capturing groups (?:...) when you don't need the captured text
  • Anchor patterns: Use ^ and $ when matching entire strings
  • Test incrementally: Build patterns step by step, testing each addition
  • Use appropriate flags: Don't use global flag if you only need the first match
  • Consider alternatives: Sometimes string methods or parsers are more appropriate than regex

Capture Groups and Advanced Features

Capture groups allow you to extract specific parts of a match. Groups are created using parentheses and are numbered starting from 1 (group 0 is the entire match). For example, in the pattern (\w+)@(\w+\.\w+), group 1 captures the username and group 2 captures the domain.

Our regex tester displays all capture groups for each match, making it easy to see what each group captured. This is particularly useful when extracting structured data from text, validating input formats, or parsing log files. When working with structured data like JSON, you can combine regex with our JSON Viewer tool to extract and validate specific fields.

Related Workflows and Tools

Standards and References

About This Tool

Maintained by the DevTools360 team. Last updated: 2026-01-15. We focus on fast, safe regex iteration with clear match visualization.

Privacy and Security

All regex testing happens entirely in your browser using JavaScript. Your patterns and test strings never leave your device, are never sent to any server, and are never stored. This privacy-first approach ensures that sensitive data like log files, user input, or confidential text remains completely private and secure. Whether you're testing regex patterns for production code, debugging text processing issues, or learning regex syntax, your data stays local and secure.

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