Regex Tester Guide
Regular expressions (regex) are powerful tools for finding and validating patterns in text. This guide covers regex basics, common patterns, and how to use online regex testers.
What is Regex?
A regular expression is a format used to find or validate specific patterns in strings. It is used for email validation, phone numbers, URLs, text replacement, and more.
Uses of Regex
- Validation: Email, password, date format validation
- Search: Extract specific patterns from log files
- Replacement: Batch text transformation
- Parsing: Extract structured data
Common Pattern Examples
1. Email Address
^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$
^: Start of string[a-zA-Z0-9._%+-]+: Local part (one or more characters)@: @ symbol[a-zA-Z0-9.-]+: Domain\.[a-zA-Z]{2,}$: Top-level domain (.com, .org, etc.)
2. Phone Number (International)
^\+?[0-9]{1,3}[- ]?[0-9]{3,4}[- ]?[0-9]{4}$
\+?: Optional + prefix[0-9]{1,3}: Country code[- ]?: Optional separator- Remaining: Local number parts
3. URL
^https?:\/\/[^\s]+$
https?: http or https:\/\/: ://[^\s]+: One or more non-whitespace characters
4. Numbers Only
^\d+$
\d: Single digit+: One or more^$: Match entire string
How to Use Online Regex Testers
UnterGletscher Regex Tester
Key features:
- ✅ Real-time match results
- ✅ Group capture display
- ✅ Multi-line test string support
- ✅ No sign-up required
- ✅ No personal data collection
Use cases:
- Debug regex patterns
- Test validation logic before implementation
- Share patterns with team members
Try it now: Use Regex Tester
JavaScript vs Python Syntax Differences
Flags
| JavaScript | Python | Description |
|---|---|---|
g | (global by default) | Global match |
i | re.IGNORECASE | Case insensitive |
m | re.MULTILINE | Multiline mode |
Usage Examples
JavaScript:
const regex = /^[a-z]+@[a-z]+\.[a-z]{2,}$/i;
regex.test("test@example.com"); // true
Python:
import re
regex = re.compile(r'^[a-z]+@[a-z]+\.[a-z]{2,}$', re.IGNORECASE)
bool(regex.match("test@example.com")) # True
Regex Writing Tips
1. Start Simple
Break complex patterns into smaller units and test each part.
2. Consider Edge Cases
Test empty strings, special characters, and long inputs.
3. Watch Performance
Patterns like .* or (.+)* can cause backtracking and slow execution.
Conclusion
Regex has a learning curve, but once mastered, it greatly improves text processing capabilities. Use UnterGletscher's free regex tester to experiment and validate your patterns.