Regex Debugger

Regex Debugger | Online Free Tool | Elysia Tools

Tool usage guide

Learn when to use this tool, what it supports, and how real users apply it.

Key facts

Category: Development
Input types: text, textarea
Output type: interactive
Sample coverage: 4
API ready: Yes

Overview

The Regex Debugger is an interactive tool designed to trace regular expression execution step-by-step. Instead of just displaying the final match, it visualizes match attempts, partial successes, backtracks, and capture groups, helping developers and learners understand exactly how a regex engine processes a test string.

When to use

When a complex regular expression fails to match or matches unexpected parts of your text.
When you need to diagnose performance issues caused by catastrophic backtracking in your regex pattern.
When learning or teaching how regex engines evaluate patterns, capture groups, and assertions position by position.

How it works

1Enter your regular expression pattern and optional flags in the designated input fields.
2Input the test text you want to evaluate against the pattern.
3Step through the interactive execution timeline to visualize how the engine attempts matches, backtracks, and captures groups at each character position.

Use cases

Debugging unexpected match failures in complex validation patterns like email or URL parsers.
Optimizing regex patterns by identifying and eliminating redundant backtracking steps.
Teaching regular expression mechanics by visualizing how greedy and lazy quantifiers behave.

Examples

1. Debugging Email Validation Backtracking

Backend Developer

Background

A developer is using a custom regex to validate email addresses, but certain long inputs cause the application to freeze due to high CPU usage.

Problem

The developer needs to find where the regex engine is getting stuck in a loop of failed matches.

How to use

Input the email regex pattern in the Pattern field, set the flags, paste the problematic email string into the Test Text field, and step through the execution trace.

Pattern: ^([a-zA-Z0-9_\-\.]+)@([a-zA-Z0-9_\-\.]+)\.([a-zA-Z]{2,5})$
Test Text: user@very-long-subdomain-name-that-fails-validation-at-the-end

Outcome

The step-by-step visualization highlights the exact position where the engine repeatedly backtracks, allowing the developer to simplify the pattern.

2. Visualizing Capture Groups in Log Parsing

DevOps Engineer

Background

An engineer needs to extract timestamps and log levels from server logs using capture groups.

Problem

The capture groups are returning empty or incorrect values for certain log formats.

How to use

FAQ

What does this debugger show that standard testers do not?

It shows the step-by-step execution path, including failed match attempts and backtracks, rather than just the final matched output.

Can I debug regex flags like global or case-insensitive?

Yes, you can specify flags such as 'g' or 'i' in the flags input field to control the matching behavior.

What is backtracking in regex?

Backtracking occurs when the regex engine hits a dead end and goes back to try alternative paths in the pattern to find a match.

Does this tool support capture groups?

Yes, it visualizes capture group assignments and matches as they occur during the step-by-step trace.

Why is my regex causing too many steps?

Nested quantifiers or overlapping patterns can cause exponential backtracking, which the debugger visualizes as repeated failed attempts.

Pattern: \[([^\]]+)\]\s+([A-Z]+)\s+(.*)
Test Text: [2026-03-30 08:00:00] ERROR Database connection failed

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Learn when to use this tool, what it supports, and how real users apply it.

Key facts

Overview

When to use

How it works

Use cases

Examples

1. Debugging Email Validation Backtracking

2. Visualizing Capture Groups in Log Parsing

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