How to Test and Debug Regular Expressions Online

Regular expressions (regex) are powerful pattern-matching tools used in programming, but they're notoriously difficult to write and debug. A single typo can make a regex fail or match the wrong text. This guide shows you how to test and debug regular expressions using fixie.tools — a free online regex tester with real-time matching, explanations, and support for JavaScript, Python, and other regex flavors.

Step 1: Open the Regex Tester

Go to fixie.tools/regex in your web browser. The tool works entirely in your browser with no server uploads or data storage. No account or signup is required.

Step 2: Enter Your Regular Expression

In the pattern field at the top, type or paste your regular expression. Don't include the surrounding slashes or delimiters — just the pattern itself. For example, enter \d{3}-\d{4} to match phone numbers like 555-1234. The tool provides syntax highlighting to help you identify different pattern components.

Step 3: Add Test Text

In the test text area, enter or paste the text you want to test your regex against. This could be sample data, log files, or any text where you want to find or validate patterns. As you type, the tool highlights all matches in real-time so you can immediately see what your regex captures.

Step 4: Select Regex Flavor and Flags

Choose your target programming language or regex flavor (JavaScript, Python, PCRE, etc.) from the dropdown, as different languages have slightly different regex syntax and features. Enable flags like case-insensitive matching (i), multiline mode (m), or global matching (g) using the checkboxes. The tool shows how these flags affect your matches.

Step 5: Review Matches and Explanation

The tool displays all matches with capture groups highlighted in different colors. Below your pattern, you'll see a plain-English explanation of what your regex does, breaking down each component. Use this to verify your regex matches what you intended and debug any issues. The match count and execution time help you optimize complex patterns.

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Frequently Asked Questions

Is the regex tester free to use?

Yes, Fixie's Regex Tester is completely free with no signup requirements or usage limits. Unlike tools like Regex101 that show ads or prompt for accounts, Fixie provides a clean, ad-free interface focused on testing your patterns.

Does the tool support all regex flavors?

The tool supports the most common regex flavors including JavaScript (ECMAScript), Python, PCRE (PHP, Perl), and Java. Each flavor has slightly different syntax and features, so selecting the right one ensures your regex will work in your target programming language.

How do I debug a regex that isn't matching?

Start by reading the plain-English explanation to verify the pattern does what you expect. Check that you're using the right flags (case-insensitive, multiline, etc.). Test with simple examples first, then gradually add complexity. The real-time highlighting shows exactly what parts of your text are being matched or missed.

Can I save and share my regex patterns?

Currently, Fixie's regex tester runs entirely in your browser without server storage, so there's no built-in save or share feature. You can bookmark the page or copy the URL (which includes your pattern in some browsers) to save your work locally.

What are capture groups and how do I use them?

Capture groups let you extract specific parts of a match by wrapping parts of your pattern in parentheses. For example, (\d{3})-(\d{4}) captures the area code and number separately in a phone number. The tool highlights each capture group in a different color so you can see what each one captures.

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