A regex tester lets you validate regular expressions against test strings in real time, showing matches, groups, and errors instantly. Whether you are a developer debugging a pattern, a data analyst validating extraction rules, or a QA engineer testing input validation, this free online regex tester gives you immediate feedback. No signup required. Try it now.
Try the Free Regex Tester
How It Works
Step 1: Enter your regex pattern
Type your regular expression pattern in the pattern field. Use standard JavaScript regex syntax including character classes, quantifiers, groups, and anchors.
Step 2: Set flags (optional)
Enter flags like g for global matching, i for case-insensitive, or m for multiline mode. The default is gi for case-insensitive global matching.
Step 3: Paste your test string
Enter the text you want to match against your pattern. The tool shows all matches with their positions and any capture groups.
How to Use the Regex Tester — Step by Step
Step 1: Write or paste your pattern
Enter a regular expression in the pattern field. Use JavaScript regex syntax: \d for digits, \w for word chars, . for any char, *, +, ? for quantifiers, () for groups.
Step 2: Choose your flags
Add flags: g (global - find all matches), i (case-insensitive), m (multiline - ^/$ match line breaks), s (dotall - . matches newlines). Default: gi.
Step 3: Provide test input
Paste the text you want to search. The tool highlights all matches, shows their positions, and extracts capture group contents.
Step 4: Debug and refine
If the pattern is invalid, the tool shows the error. Adjust the pattern and test again until matches are correct. The error message helps you pinpoint syntax issues.
Common Use Cases
- Input Validation: Test email, phone, URL, credit card, and custom format validators before deploying to production — catch regex bugs before they reach users
- Data Extraction: Build patterns to extract emails, dates, IDs, prices, or structured data from logs and documents — verify capture groups extract exactly what you need
- Log Parsing: Create patterns to parse application logs, access logs, or error traces into structured fields for analysis and alerting
- Code Refactoring: Test patterns for find-and-replace operations across codebases (variable renaming, API changes) — preview changes before applying
- Learning Regex: Experiment with patterns interactively — see exactly what matches and what groups capture, build intuition for regex syntax
- Security Auditing: Test patterns for detecting sensitive data (API keys, passwords, PII) in code or logs before deployment
Examples
Pattern: [\w.-]+@[\w.-]+\.\w+ | Flags: gi
Input: "Contact [email protected] or [email protected] for help"
Output: 2 matches: "[email protected]" (index 8), "[email protected]" (index 33)
Pattern: (\d{3})-(\d{4}) | Flags: g
Input: "Call 555-1234 or 555-5678 today"
Output: 2 matches: "555-1234" (Group 1: 555, Group 2: 1234), "555-5678" (Group 1: 555, Group 2: 5678)
Pattern: (\d{4}-\d{2}-\d{2})\s+(\w+)\s+(ERROR|WARN) | Flags: g
Input: "2024-01-15 10:00:00 Auth ERROR\n2024-01-15 10:00:01 DB WARN"
Output: 2 matches with 3 capture groups each (date, component, level)
Pattern: (? | Flags: g
Input: "Prices: $19.99, $1,299.00, $50"
Output: 3 matches with named capture group "currency" extracting each price
Tips & Best Practices
- Start simple: Build complex patterns incrementally — test each piece before combining into the full pattern
- Use capture groups: Parentheses
()extract parts for replacement or validation — named groups(?improve readability...) - Escape special chars:
.*+?^${}[]()|\need\to match literally - Test edge cases: Empty strings, boundaries, special characters, unicode — verify your pattern handles them correctly
- For replacements: Use Regex Find/Replace tool with
$1,$2for group references — named groups use$ - Performance: Avoid catastrophic backtracking — use possessive quantifiers or atomic groups for complex patterns on large inputs
- Anchors: Use
^and$(withmflag) to match line boundaries — critical for validation patterns
FAQ — Frequently Asked Questions
Q: What is a regular expression?
A: A regular expression (regex) is a pattern used to match character combinations in strings. It's a powerful tool for text search, validation, and manipulation.
Q: What flags can I use?
A: Common flags: g (global — find all matches), i (case-insensitive), m (multiline), s (dotall). Enter them in the flags input field.
Q: What regex flavor does this use?
A: This tool uses JavaScript's native RegExp engine (ECMAScript regex flavor). It supports character classes, quantifiers, groups, lookaheads, and all standard JS regex features.
Q: Can I test replacement patterns?
A: This tool shows matches and capture groups. For find-and-replace testing, use the Regex Find/Replace tool which supports replacement patterns with $1, $2, etc.
Q: Is the tool secure?
A: Yes. All processing happens in your browser. We never see, store, or save your text or patterns.