Regex Tester
Write and test regex patterns with instant match highlighting, flag toggles (global, case-insensitive, multiline, dotall), capture group display, and match index tracking. All processing in your browser.
Validation and extraction
Test patterns against sample payloads before using them in forms, ETL jobs, or app logic.
Flag-aware debugging
Toggle global, case-insensitive, multiline, and dotall behavior to see exactly why a match succeeds or fails.
Capture group inspection
Review extracted groups and start positions before you ship a parser, validator, or search rule.
Build and verify a pattern locally in the browser before copying it into application code, form rules, or search logic.
Large inputs are safely trimmed for performance after 50,000 characters.
Match count
0
Active flags
g
Pattern state
Add a pattern
What This Regex Tester Is Best For
Use this page when you need to validate a regular expression, inspect capture groups, or compare how flags change the result before you move the pattern into production code. It is especially useful for parsing logs, validating form inputs, extracting fields from text, and debugging search rules.
Pattern Validation
Catch invalid syntax and flag mistakes before the pattern reaches app code, automation, or a form validator.
Capture Group Debugging
See what each group actually extracts so replacements, parsers, and downstream transforms are easier to trust.
Flag Testing
Compare global, multiline, case-insensitive, and dotall behavior against realistic sample text instead of guessing.
How to Use the Regex Tester
- Start with one of the built-in examples or paste your own regex pattern
- Toggle flags such as g, i, m, and s to mirror your real runtime behavior
- Paste a sample string, payload, log line, or block of text into the test area
- Review the highlighted matches, total match count, and any syntax errors immediately
- Check capture groups and match indexes before copying the pattern into code or a validator
Regex Best Practices
Test With Realistic Input
A pattern that works on a short demo string can still fail on multiline logs, escaped characters, or unexpected whitespace in production.
Use Flags Deliberately
The same pattern can behave very differently with or without global or multiline matching. Verify the exact combination you plan to ship.
Keep Capture Groups Intentional
Only capture the pieces you actually need. Extra groups make replacement logic and downstream parsing harder to maintain.
Validate Before Embedding
Before you add a regex to app code, CMS rules, or data cleanup scripts, confirm it matches the intended values and nothing else.
Common Regex Patterns
Related Developer Workflows
JSON Formatter
Format payloads first, then test regex patterns against clearer nested values and string fields.
Diff Checker
Compare two text versions after a regex cleanup or replacement step to verify what changed.
Base64 Encode & Decode
Decode tokens or encoded payloads before trying to extract values with regular expressions.
Timestamp Converter
Useful when your regex pulls Unix timestamps out of logs, events, or exported datasets.
What Are Regular Expressions?
Regular expressions (regex) are sequences of characters that define search patterns. Rooted in formal language theory and finite automata, they provide a concise syntax for matching, extracting, and manipulating text. Nearly every programming language includes a regex engine, making them an indispensable tool for developers.
A regex pattern can range from a simple literal string to a complex expression with quantifiers, character classes, lookaheads, and capture groups. While powerful, regex can be difficult to read and debug, which is why a live tester with instant highlighting is invaluable for building and validating patterns.
Common Use Cases
Input Validation
Validate formats like email addresses, phone numbers, postal codes, and credit card numbers on both client and server sides.
Log Parsing
Extract timestamps, error codes, IP addresses, and other structured data from unstructured or semi-structured log files.
Search and Replace
Perform powerful find-and-replace operations in text editors and build scripts using capture groups and backreferences.
Data Extraction and Scraping
Pull specific fields from HTML, CSV, or plain text when a full parser is not necessary or available.
Tips & Best Practices
Avoid Catastrophic Backtracking
Nested quantifiers like (a+)+ cause exponential backtracking. Use atomic groups or possessive quantifiers to keep performance predictable.
Use Non-Capturing Groups
Write (?:...) instead of (...) when you do not need the matched text. This improves clarity and can boost engine performance.
Test Edge Cases Thoroughly
Always test with empty strings, extremely long input, special characters, and unicode. Regex that works on happy paths often fails on edges.
Prioritize Readability
Use the verbose/extended flag (x) to add comments and whitespace. A regex others can read is far more valuable than a clever one-liner.