Regex Tester: Test Regular Expressions with Live Matching
Test and debug regular expressions with live match highlighting, capture group breakdowns, replace preview, and protection against catastrophic backtracking.
Common Regex Patterns Library
//
Substitution / Replace With:
Match Results (2 matches found)
#1support@formatflow.site(index 14)
#2feedback@formatflow.site(index 41)
Substitution Preview
Contact us at [REDACTED_EMAIL] or [REDACTED_EMAIL] for developer inquiries. Our development server is running on https://formatflow.site/docs/api since 2026-10-25. IPv4 Gateway: 192.168.1.1, Primary DNS: 8.8.8.8.
How to Use Regex Tester
- Enter your regular expression pattern and select your desired flags (g = global, i = case insensitive, m = multiline, s = dotAll).
- Or choose from our Common Patterns Library (Emails, URLs, IP addresses, Dates, UUIDs).
- Paste your target test text in the middle box.
- View live match counts, match positions, captured sub-groups, and replacement preview below.
Practical Examples
Extracting Email Addresses
Global matching of all RFC-compliant email strings.
Input:
Pattern: [\w.]+@[\w.]+\.[a-z]{2,}Output:
Matches: support@formatflow.site, feedback@formatflow.site
Catastrophic Backtracking Protection
Invalid or catastrophic patterns are safely aborted with timeout protection.
Input:
Pattern: (a+)+$
Output:
Guarded with timeout safety
Common Mistakes & Troubleshooting
- Missing the global flag (g): Without "g", JavaScript RegExp only stops after finding the first single match.
- Unescaped special characters: Characters like . * + ? ^ $ ( ) [ ] { } | \ must be escaped with a backslash if meant literally.
Frequently Asked Questions
What happens if a regex causes catastrophic backtracking?
FormatFlow includes an internal performance timeout cap that halts long-running evaluations after 300ms, preventing browser tab freezes.
Which flags are supported?
We support all standard ECMAScript regex flags: g (global), i (case insensitive), m (multiline), s (dotAll), and u (unicode).