Regex Tester – Test Regular Expressions with Live Matches
Test regular expressions with real-time matching and highlighting.
How to use this regex tester
- Enter your regular expression in the "Regular Expression Pattern" field (without the surrounding slashes)
- Toggle the Global (g), Case-insensitive (i), and Multiline (m) checkboxes to control how the pattern matches
- Paste or type your sample text into the "Test String" area
- View every match highlighted in yellow within the test string as you type
- Check the match count and the raw match details shown below the highlighted text
- If the pattern is invalid, read the error message shown under the pattern field and adjust your regex
Frequently Asked Questions
How does regex testing work?
Enter a regular expression pattern and a test string. The tool will show all matches in real-time and highlight them in the test string.
What regex flags are supported?
The tool supports global (g), case-insensitive (i), and multiline (m) flags. These modify how the regex pattern matches the test string.
Can I test complex regex patterns?
Yes, the tool supports all JavaScript regex features including groups, lookaheads, quantifiers, and character classes. Invalid patterns will show an error message.
A regex tester helps developers, QA engineers, and data analysts write and debug regular expressions against real sample text before dropping them into production code. This tool is useful when building form validation, writing log-parsing scripts, extracting data from text, or figuring out why a regular expression isn't matching what you expect. Instead of running a script repeatedly to check a pattern, you can type a regex and a test string side by side and immediately see every match highlighted inline. This is especially handy for iterating on tricky patterns involving groups, quantifiers, or lookaheads, where small changes can significantly affect what gets matched. All matching happens instantly in your browser as you type, with no need to install a regex library or run any code.
What is regex tester?
A regex tester works by compiling the pattern you enter into a native JavaScript RegExp object, combined with whichever flags you enable, and running it against your test string using JavaScript's built-in match() and exec() methods. When the global flag is enabled, the tool repeatedly calls exec() to find every match in the text and highlights each one inline; without it, only the first match is found. If the pattern is syntactically invalid, constructing the RegExp throws an error, which the tool displays instead of results. Because this uses the browser's native regex engine, the exact matching behavior follows the JavaScript (ECMAScript) regex flavor rather than other implementations like PCRE, Python's re module, or POSIX regex, which can differ in how they handle certain constructs.
Common use cases
- Building and verifying a validation pattern for emails, phone numbers, or postal codes before adding it to a form
- Debugging why a log-parsing regex isn't capturing the expected fields from sample log lines
- Testing a find-and-replace pattern before running it across a codebase or dataset
- Checking how a pattern behaves with different flag combinations, such as case-insensitive or multiline matching
- Extracting specific substrings (like hashtags, URLs, or IDs) from sample text during data cleaning
- Verifying a regex used in routing rules, redirects, or URL rewriting behaves as intended
Limitations and common mistakes
- Matching uses the JavaScript (ECMAScript) regex engine, so behavior can differ from PCRE, Python, or POSIX regex flavors — some lookbehind, Unicode, or possessive-quantifier features may not behave identically
- Only the global (g), case-insensitive (i), and multiline (m) flags are exposed; other JavaScript flags like dotAll (s), unicode (u), or sticky (y) cannot be toggled in this tool
- Without the global flag enabled, only the first match is found and highlighted, even if the pattern occurs multiple times in the test string
- Catastrophic backtracking patterns can slow down or momentarily freeze the browser tab, since matching runs synchronously on the main thread
- The match details are shown as a raw JSON dump of the match array, which may be verbose for patterns with many capture groups