Regex Tester – Free Online Regular Expression Tester & Debugger Tool

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Regex Flags
Enter a regex pattern and test string to see highlighted matches
No matches to display
No replacements to display

About Regex Tester

The Regex Tester is an online tool designed to help developers test and validate regular expressions against a custom test string. It allows you to enter a regex pattern, apply supported flags, and instantly see how the expression behaves.

This tool is ideal for developers, QA engineers, and technical users who need to:

  • Verify pattern accuracy

  • Inspect match positions

  • Analyze capture groups

  • Preview replacement behavior

All results are displayed clearly in structured tabs, making it easy to debug and refine your regular expressions.

How the Regex Tester Works

Using the tool is straightforward:

Step 1: Enter a Regular Expression Pattern

Input your regex pattern in the Pattern field.

Step 2: Enter a Test String

Provide the text you want to evaluate against your regex.

Step 3: Select Regex Flags (Optional)

Choose from the available flags:

  • Ignore Case (i)

  • Multiline (m)

  • Dot All (s)

  • Unicode (u)

  • Verbose (x)

Step 4: Click “Test Regex”

The tool sends a structured POST request to the backend and processes your pattern using Python’s re engine.

Step 5: View Results in Three Tabs

Highlight Tab
Visually highlights all matched segments within the test string.

Matches Tab
Displays:

  • Matched text

  • Start and end index positions

  • Captured groups

  • Named groups (if defined)

Replacements Tab
Shows two automatic replacement previews:

  1. Simple replacement (all matches replaced with REPLACEMENT)

  2. Group-based replacement (outputs MATCH(g1, g2, ...) for captured groups)

Supported Regex Flags

This tool supports the following Python re flags:

Flag Name Behavior
i Ignore Case    Case-insensitive matching
m Multiline   ^ and $ match line boundaries
s Dot All   . matches newline characters
u Unicode     Enables Unicode-aware matching
x Verbose     Allows whitespace and comments inside pattern

These flags are applied directly during regex compilation.

Match Results Explained

When a regex successfully matches, the tool returns detailed match information:

  • Match Text – The exact substring matched.

  • Start Position – Index where the match begins.

  • End Position – Index where the match ends.

  • Capture Groups – Values captured by parentheses.

  • Named Groups – Values captured using named group syntax.

All matches are detected using Python’s finditer() method, ensuring accurate position tracking.

Replacement Preview Features

The tool automatically generates two replacement previews:

1. Simple Replacement

Every match is replaced with:

REPLACEMENT

This shows how the string would look after a global substitution.

2. Group-Based Replacement

If capturing groups exist, matches are replaced with:

MATCH(group1, group2, ...)

If no groups exist, matches are replaced with:

MATCH

This helps visualize how captured values can be used during substitution.

Technical Details

  • Backend built with Django.

  • Regex processing powered by Python’s re module.

  • Uses re.compile() with selected flags.

  • Matches retrieved via re.finditer().

  • Replacements performed using re.sub().

  • Structured JSON response includes:

    • Match list

    • Capture groups

    • Named groups

    • Replacement previews

    • Validation status

Invalid regular expressions are safely caught and returned with an error message.

Error Handling

The tool handles errors in two layers:

Frontend Validation

  • Requires both pattern and test string.

  • Displays user-friendly notifications.

Backend Validation

  • Detects invalid regex syntax.

  • Returns detailed error messages.

  • Prevents application crashes.

If a pattern is invalid, the results section displays the specific regex error.

Use Cases

This Regex Tester is useful for:

  • Debugging input validation patterns

  • Testing data extraction regex

  • Verifying capturing group behavior

  • Checking match index positions

  • Previewing global substitutions

  • Experimenting with multiline or case-insensitive patterns

It is especially helpful during backend development where Python-style regular expressions are used.

Why This Tool Is Useful

  • Instant visual feedback

  • Accurate match indexing

  • Clear group inspection

  • Structured replacement previews

  • Clean tab-based interface

  • No installation required

The interface updates results dynamically and provides clear formatting for technical analysis.

Data & Privacy

This tool does not store, save, or log your regex patterns or test strings.

All data is processed through a POST request and returned immediately as a JSON response. No database storage or file persistence is involved in the process.

Browser & Device Compatibility

  • Works in modern browsers

  • Responsive layout for desktop and mobile

  • Supports dynamic tab switching

  • Displays highlighted matches with safe HTML escaping

The interface adjusts automatically for smaller screens.

Regex Tester FAQs

Frequently asked questions about using our Regex Tester tool.

This tool uses Python’s built-in re module to compile and test regular expressions. All matching and replacements follow Python regex behavior.

Yes. The tool supports the following flags:

  • Ignore Case (i)

  • Multiline (m)

  • Dot All (s)

  • Unicode (u)

  • Verbose (x)

These flags are applied during regex compilation.

Yes. For every match found, the tool displays:

  • The matched text

  • The start index

  • The end index

This helps you understand exactly where the match occurs in your test string.

Yes. If your regex contains capturing groups, the tool displays:

  • Group values (by index)

  • Named groups (if defined in the pattern)

If no groups exist, only the full match is shown.

No. The tool does not allow custom replacement input. It automatically generates two predefined replacement previews:

  • A simple replacement using the text REPLACEMENT

    • A group-based replacement using MATCH(...)

If the pattern contains invalid syntax:

  • The backend catches the error safely.

  • The tool displays the specific error message.

  • No matches or replacements are processed.

No. The tool does not store or save your data. Your pattern and test string are processed in a POST request and immediately returned as a response without database storage.

Yes. The test string input supports multiline text. When the Multiline flag is enabled, ^ and $ will match line boundaries accordingly.

There is no explicit size limit defined However, performance depends on the complexity of your regex pattern and the length of the test string.

Yes. All detected matches are highlighted directly inside the test string in the Highlight tab for easy visual inspection.

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