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ToolZilla

Test & Debug Regular ExpressionsMatch · Replace · Generate Code

Test regex patterns in real time with match highlighting, replace mode, cheat sheet, and code generation for JavaScript & Python.

Regular Expression
//g
Test String

Quick Patterns

Why Use Our Regex Tester

Debug patterns faster than ever

🎯

Real-Time Matching

See matches highlighted instantly as you type your pattern

🔄

Replace Mode

Test find-and-replace operations with $1/$2 group reference support

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Cheat Sheet

Built-in reference for characters, anchors, quantifiers, and groups

💻

Code Generation

Export working regex code snippets for JavaScript and Python

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Quick Patterns

Pre-built patterns for email, URL, phone, IP address, and more

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Group Capture

Visualize captured groups with match indices and detailed breakdown

Complete Guide: How to Use the Regex Tester

Test and debug regular expressions in real-time with instant highlighting, capture group visualization, and match details. Our regex tester supports JavaScript regex syntax and shows you exactly what your pattern matches, making it essential for developers working with text processing, validation, or data extraction.

Step-by-Step Instructions

  1. 1

    Enter your regex pattern

    Type your regular expression in the pattern field. The tool validates the syntax in real-time and highlights any errors immediately.

  2. 2

    Set flags (optional)

    Toggle regex flags: g (global), i (case-insensitive), m (multiline), s (dotAll). Each flag changes how the pattern matches.

  3. 3

    Enter your test string

    Paste or type the text you want to match against. Matches are highlighted instantly in the test string as you type.

  4. 4

    Review matches and groups

    See all matches listed with their index positions and capture groups. Click any match to see its details.

Common Use Cases

  • ✓Form validation — build and test patterns for email, phone, URL, and date validation
  • ✓Data extraction — craft regex to pull specific data from logs, CSVs, or HTML
  • ✓Search and replace — test find/replace patterns before running them on production data
  • ✓Log analysis — filter and extract information from server logs and error messages
  • ✓Text processing — build patterns for tokenization, parsing, and text transformation
  • ✓Learning — visualize how regex engines match patterns step by step

Pro Tips

💡Use non-capturing groups (?:...) when you need grouping but don't need to capture the match — it's more efficient.
💡The .* pattern is greedy by default — it matches as much as possible. Use .*? for lazy (minimal) matching.
💡Always test your regex with edge cases: empty strings, very long inputs, and strings that almost-but-don't-quite match.
💡Named capture groups (?<name>...) make your regex more readable and maintainable than numbered groups.

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Frequently Asked Questions

What is a regular expression (regex)?

A regular expression is a sequence of characters that defines a search pattern. It's used for pattern matching within strings — common uses include validation (emails, phone numbers), search-and-replace, and text parsing. Most programming languages support regex natively.

What do the common regex flags mean?

The most common flags are: g (global) — find all matches instead of stopping at the first; i (case insensitive) — ignore upper/lower case; m (multiline) — treat each line as a separate string for ^ and $; s (dotAll) — makes the dot (.) match newline characters as well.

How do I match and extract groups in regex?

Use parentheses () to create capturing groups. For example, (\d{3})-(\d{4}) on '555-1234' captures '555' as group 1 and '1234' as group 2. Named groups use the syntax (?<name>...). Access groups via the match result array.

What are some common regex patterns?

Common patterns include: email validation (\S+@\S+\.\S+), URLs (https?://[^\s]+), phone numbers (\(?\d{3}\)?[-.\s]?\d{3}[-.\s]?\d{4}), dates (\d{4}-\d{2}-\d{2}), IP addresses (\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}), and hex colors (#[0-9A-Fa-f]{3,8}).

Why is my regex not matching what I expect?

Common issues include: forgetting the global flag (g) so only the first match is returned, not escaping special characters like . or $, using greedy quantifiers (* or +) when you need lazy ones (*? or +?), and anchoring issues with ^ and $ in multiline strings.