Regex Tester
Test regular expressions, see matches & groups, replace text, use ready‑made patterns, and keep history — all in one place. Completely private and client‑side. No data ever leaves your device.
Match Details & Groups
| # | Match | Index | Groups |
|---|
Find & Replace
Regex History
What is a Regular Expression?
A regular expression (regex) is a sequence of characters that defines a search pattern. It's used for pattern matching within strings — to find, validate, extract, or replace text based on specific rules. Regex is supported in almost every programming language and is an essential tool for developers, data analysts, and anyone working with text.
For example, the pattern \d{3}-\d{2}-\d{4} matches a US Social Security Number like 123-45-6789. Regular expressions can be simple (like finding all occurrences of "cat") or incredibly complex (matching nested structures, email addresses, or URLs).
Why Use Our Online Regex Tester?
- Instant feedback: See matches highlighted as you type or with a single click. No need to set up a programming environment.
- Privacy‑first: All processing happens in your browser. Your text and patterns never leave your device.
- Complete feature set: Test with various flags, see capture groups (including named groups), replace text, copy results, and view history.
- Cross‑platform: Works on Windows, Mac, Linux, iOS, and Android — anything with a modern browser.
- Free forever: No subscriptions, no signups, no ads that interfere with the tool.
How to Use the Regex Tester
- Enter your text – Paste the text you want to search through.
- Choose a pattern – Select a common regex from the dropdown or write your own.
- Adjust flags – Toggle case‑insensitive, global, multiline, dot‑all, and unicode options.
- Click "Test Regex" – Instantly see highlighted matches, match count, and capture groups.
- Replace (optional) – Use the replace section to substitute matches with new text.
- Enable Live Update – See results as you type without clicking the button.
- Browse history – Quickly reload any of your previous patterns from the history panel.
Regular Expression Quick Reference
| Pattern | Description |
|---|---|
. | Any character except newline |
\d | Any digit (0-9) |
\w | Word character (a-z, A-Z, 0-9, _) |
\s | Whitespace (space, tab, newline) |
+ | One or more of the preceding character |
* | Zero or more |
? | Zero or one (optional) |
{n,m} | Between n and m occurrences |
^ | Start of string (or line with m flag) |
$ | End of string (or line with m flag) |
[abc] | Any character inside brackets |
(abc) | Capture group |
(?<name>...) | Named capture group |
Common Regex Patterns Explained
| Use Case | Pattern | Explanation |
|---|---|---|
[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,} | Matches typical email addresses. | |
| URL | https?://[^\s/$.?#].[^\s]* | Matches HTTP/HTTPS URLs. |
| US Phone | \b\d{3}[-.]?\d{3}[-.]?\d{4}\b | Matches 123-456-7890 or 123.456.7890. |
| Date (YYYY-MM-DD) | \d{4}-\d{2}-\d{2} | Matches 2024-12-31 format. |
| IPv4 Address | \b\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}\b | Matches simple IP addresses. |
| Hex Color | #[0-9a-fA-F]{6} | Matches #ff0000, #00FF00, etc. |
| UUID | \b[0-9a-fA-F]{8}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{12}\b | Matches standard UUIDs. |
Real‑World Use Cases
- Validate emails: Quickly test that an email pattern matches sample data.
- Extract phone numbers: Pull phone numbers from a large document.
- Redact sensitive data: Replace credit card numbers or SSNs with ***.
- Reformat dates: Use capture groups to rearrange date formats (e.g., YYYY‑MM‑DD to DD/MM/YYYY).
- Clean code: Identify patterns like trailing spaces, unused imports, or specific function names.
Tips for Writing Effective Regex
- Start simple: Build your regex step by step, testing each part individually.
- Use character classes:
[aeiou]is cleaner than writing out each vowel. - Escaping special characters: Characters like
.,+,*, and?need a backslash if you mean them literally. - Non‑capturing groups: Use
(?:...)when you don't need to extract a group. - Whitespace awareness: Remember that
\smatches spaces, tabs, and newlines. Use the multiline flag wisely.
Troubleshooting Common Errors
- "Nothing matches": Check if your flags are correct (especially case‑insensitive and global).
- "Catastrophic backtracking": Complex patterns with nested quantifiers can hang. Simplify or use atomic groups.
- "Pattern invalid": Ensure all parentheses and brackets are balanced, and special characters are escaped.
- "Unexpected matches": Use anchors
^and$to avoid partial matches in the middle of strings.
Pro Tip: When writing complex patterns, test step‑by‑step. Start with a small part of the regex, test it, and gradually add more. The live update feature helps you catch mistakes early.
Frequently Asked Questions
No. Everything runs locally in your browser. Your text and regex patterns are never uploaded.
Yes, the tool uses JavaScript's RegExp engine which supports Unicode patterns. Enable the Unicode flag (u) and use
\p{L} style escapes.The tool works with one pattern at a time. For multiple patterns, you can run them sequentially or combine them using alternation (
|).The tool will display an error message if the pattern is syntactically incorrect, helping you debug it.
It uses JavaScript's
String.replace() with your regex flags. You can use $1, $2 to reference capture groups.Absolutely. The page is fully responsive and works on all modern devices.
Yes, history is stored only in your browser's local storage and never transmitted.
Yes! Because the tool runs entirely in your browser, you can save this page (Ctrl+S) and use it without an internet connection.
There are no arbitrary limits. However, very large text (hundreds of MB) may slow down your browser. For typical use cases, it works perfectly.