Regex Tester - Regular Expression Tester & Debugger
Regular expressions are an indispensable tool for text validation, scraping, and string transformations. However, complex patterns involving lookaheads, lookbehinds, and nested capture groups can easily lead to greedy matching or catastrophic backtracking. This interactive tester evaluates your regular expression in real time as you type, highlighting all match occurrences and separating indexed capture groups.
When to Use This Tool
- Form Input Validation: Craft bulletproof patterns for email addresses, international phone numbers, and strong passwords.
- Server Log Extraction: Extract IP addresses, HTTP status codes, and request URIs from Nginx or Apache access logs.
- Data Cleaning & Scraping: Extract HTML attributes, markdown links, or monetary values from unorganized raw text.
- Capture Group Verification: Verify that numbered ($1, $2) or named capture groups isolate the exact intended substrings.
How to Use
- Enter Pattern: Type your regular expression pattern in the regex bar (e.g., [a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}).
- Set Flags: Toggle flags: g (global), i (case-insensitive), m (multiline), and s (dotAll).
- Input Test String: Paste sample text to test matches against.
- Inspect Matches: View highlighted text spans, match count, and broken-down capture groups.
Input & Output Example
Input:
Pattern: (\d{4})-(\d{2})-(\d{2})
Text: Project launched on 2026-09-04 successfully.
Output:
Full Match: "2026-09-04"
Group 1: "2026"
Group 2: "09"
Group 3: "04"
Frequently Asked Questions (FAQ)
Q. Which regex dialect does this tool support?
A. It runs the ECMAScript (JavaScript) standard regex engine natively provided by modern web browsers, supporting modern features like Unicode property escapes and lookaround assertions.
Q. What is Catastrophic Backtracking?
A. When nested quantifiers (e.g., (a+)+$) fail to match, the regex engine attempts an exponential number of permutations, freezing the execution. If a pattern takes too long, your browser sandbox protects against system crashes.
Q. How can I replace matches?
A. You can test substitution patterns using standard replacement tokens like $1 and $2 to verify your output before implementing in code.
Engineering & Privacy Notes
- Evaluated natively via the V8/SpiderMonkey JavaScript regex compiler for authentic runtime behavior.
- No data leaves your device; test patterns on proprietary logs with zero confidentiality risk.
Related Developer Tools
- URL Encoder: Encode and decode URL parameters verified via regex.
- Diff Checker: Compare text differences before and after regex replacements.
- Text Analyzer: Analyze character count and statistics of matched text.
- JSON Formatter: Format JSON strings extracted via regex parsing.