Regex Tester
Safely test regular expressions in real-time in your browser.
Supports match highlighting and capture group extraction.
※ You can use capture groups (e.g., $1, $2).
🔒Text is never sent to a server.
About the Regex Tester
The Regex Tester is a free developer tool that allows you to test and verify whether your Regular Expression (Regex) pattern matches the text as intended in real-time.
It runs the JavaScript (ECMAScript) regex engine directly in your browser and supports toggling flags like `g` (global), `i` (ignoreCase), and `m` (multiline). Furthermore, it provides replacement simulation using capture groups (e.g., `$1`, `$2`).
How to Use
Enter Regex
Enter the pattern you want to test. Turn on flags like g, i, m as needed.
Enter String
Paste the target text to be searched into the left input area.
Check Matches
Parts matching the pattern will be highlighted in real-time in the right area.
Test Replacement
Turn on "Use Replace Feature" and enter a replacement string to preview the replacement result.
Regex Glossary
- Regular Expression
- A sequence of characters that specifies a search pattern in text. Widely used for validation (e.g., email format) and string extraction/replacement.
- Flags
- Execution options for regex. `g` (Global) searches for all matches, `i` (IgnoreCase) makes matches case-insensitive, and `m` (Multiline) searches in multiline mode.
- Capture Group
- Enclosing part of a pattern in parentheses `()` allows you to reuse the matched portion later as variables like `$1`, `$2` in replacements.
- Escape
- Prefixing a backslash `\` to treat characters with special meaning in regex (like `.`, `*`, `?`) as literal characters.
- Lookahead / Lookbehind
- Advanced regex syntax that asserts whether a specific pattern is preceded or followed by another pattern.
Frequently Asked Questions
- Q.Which regex engine is supported?
- This tool uses the standard JavaScript (ECMAScript) regular expression engine. Therefore, it may not perfectly match PCRE (PHP, etc.) or Python-specific regex behaviors (such as some named capture group behaviors).
- Q.Why are the match results not highlighted?
- If there is a syntax error in your pattern (like unclosed parentheses), an error message will appear and highlighting will be disabled. Additionally, if the g (Global) flag is off, only the first match will be highlighted.
- Q.Is it safe to test text containing sensitive information (passwords, customer data)?
- Yes, it is perfectly safe. This tool processes everything entirely within your browser (client-side), and neither your entered text nor your regex patterns are ever sent to external servers.
- Q.How can I use capture group values when replacing?
- By using formats like $1 and $2 in the replacement string, you can insert the text matched by the first and second capturing groups () in your regex pattern.
- Q.Can the process become slow or freeze?
- Please be aware that running a poorly constructed regex on very long or complex text can cause "catastrophic backtracking," which may make your browser unresponsive.
Use Cases
Creating Input Form Validation
Build regex for validating email addresses, phone numbers, or zip codes on user registration screens, and test them thoroughly against various test cases.
Log File Parsing and Extraction
Use it as a sandbox when constructing regex to extract specific error codes or IP addresses from server access logs.
Code Refactoring
Safely test replacement patterns involving complex capture groups to globally replace specific variable names or function call formats in source code.
Text Cleansing
Simulate replacement patterns in advance to safely remove full-width spaces, extra line breaks, or unwanted HTML tags.
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