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Regex Railroad Diagram Visualizer

· Last updated：2026-06-20

Render a regular expression as an SVG railroad (syntax) diagram showing literals, groups, alternations, and repetition, with capture-group numbering and flag explanations

Details

What this tool helps you do

Parses a JavaScript/PCRE-flavored regex with the ret library into an AST, then maps each node to a railroad-diagram primitive (rounded boxes for literals, labeled groups for captures, choice branches for alternations, loop-back arrows for quantifiers) rendered as SVG via railroad-diagrams. Also lists every capture group (with name) and explains each flag. Useful for teaching regex, documenting patterns, and spotting nested-quantifier backtracking risks.

Execution

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Written by the Elysia Tools team
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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
Output type
html
Sample coverage
4
API ready
Yes

Overview

The Regex Railroad Diagram Visualizer converts complex JavaScript and PCRE-flavored regular expressions into clean, interactive SVG syntax diagrams. By parsing your regex pattern into an Abstract Syntax Tree (AST), the tool visually maps literals, capture groups, alternations, and quantifiers into an easy-to-read flowchart, complete with capture-group numbering and flag explanations.

When to use

  • When documenting complex regular expressions in technical specifications or codebase readmes.
  • When debugging nested quantifiers, alternations, or capture groups to prevent performance issues like catastrophic backtracking.
  • When teaching or learning regular expression syntax and wanting to visualize how the engine parses a pattern.

How it works

  1. 1Enter your JavaScript or PCRE-compatible regular expression pattern and optional flags into the input fields.
  2. 2The tool parses the pattern into an Abstract Syntax Tree (AST) using the ret library.
  3. 3Each AST node is mapped to a visual railroad primitive, such as rounded boxes for literals, choice branches for alternations, and loop-back arrows for quantifiers.
  4. 4The visualizer outputs a self-contained SVG diagram along with a detailed list of numbered capture groups and flag explanations.

Use cases

  • Visualizing complex URL routing patterns to verify capture groups and optional parameters.
  • Documenting email validation patterns in team wikis using clear, graphical syntax diagrams.
  • Auditing legacy regex patterns for nested loops that could cause performance bottlenecks.

Examples

1. Visualizing a URL-matching regex

Backend Developer

Background

A developer needs to document a complex URL routing regex pattern for an API gateway to ensure team members understand the path parameters.

Problem

The regex pattern is difficult to read and explain in plain text.

How to use

Input the URL pattern into the Regular Expression field and set the flags to i.

Pattern: ^(https?)://([^/:]+)(:\d+)?(/.*)?$
Flags: i

Outcome

An SVG diagram is generated showing clear branches for optional protocols, hostnames, ports, and paths, alongside a list of the four numbered capture groups.

2. Deconstructing an Email Capture Pattern

QA Engineer

Background

A QA engineer wants to verify that an email validation regex correctly captures the domain and username parts without backtracking issues.

Problem

Verifying the boundary conditions and character classes of a complex email regex manually is error-prone.

How to use

Paste the email regex pattern into the input field and add the gi flags.

FAQ

What regex flavors are supported?

The visualizer supports JavaScript and PCRE-flavored regular expressions.

Can it identify named capture groups?

Yes, it lists and labels all capture groups, including named groups and their corresponding indices.

Does the tool support advanced PCRE features like recursion?

No, advanced features like recursive patterns, branch resets, and conditional groups are not supported and will return a parsing error.

How do I read the generated railroad diagram?

Read the diagram from left to right, following the paths through literals, branches, and loop-back arrows for repetitions.

Can I export the diagram?

Yes, the diagram is rendered as a standard SVG element that you can copy or save directly from your browser.

Hub

Regex Testing, Visualization, and Safety Workflow

Build reliable regular expressions by learning syntax, testing matches, visualizing structure, previewing replacements, benchmarking alternatives, and scanning for ReDoS risk.

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