How to Build a Regression Suite for Legacy Apps Without Accessing Source Code

You have been handed a 15-year-old enterprise portal. The original developers are long gone, the documentation is a single outdated PDF, and the source code is a tangled mess of undocumented logic that no one dares to touch. Every time a minor change is made to the environment, the entire UI breaks in unpredictable ways. You need a safety net—a way of building regression suite legacy workflows—but you don't even have access to the underlying repository or the ability to recompile the app.

This is the reality of the $3.6 trillion global technical debt. Traditional testing requires deep hooks into the code, but when the code is a "black box," those tools fail. You need a method that ignores the "how" and focuses entirely on the "what."

TL;DR: Building a regression suite for legacy applications without source code requires Visual Reverse Engineering. By using Replay (replay.build), teams can record user sessions to automatically generate pixel-perfect React components and Playwright/Cypress E2E tests. This "Record → Extract → Modernize" workflow reduces manual effort from 40 hours per screen to just 4 hours, providing a 10x context advantage over traditional screenshots.

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What is the best way to build a regression suite for legacy apps?#

The most effective way to build a regression suite for a legacy application without source code access is through Behavioral Extraction. Instead of trying to read the code, you record the application's behavior.

Video-to-code is the process of converting screen recordings of a user interface into functional, production-ready code and automated tests. Replay pioneered this approach, allowing architects to capture the temporal context of an application—how buttons hover, how modals transition, and how data flows—without needing to see a single line of the original backend logic.

According to Replay’s analysis, 70% of legacy rewrites fail or exceed their timelines because the team lacks a baseline of existing behavior. By building regression suite legacy assets through video, you create a "Source of Truth" that exists independently of the legacy codebase.

The Replay Method: Record → Extract → Modernize#

1. •Record: Capture a user performing a critical path (e.g., "Create New Invoice").
2. •Extract: Replay’s AI analyzes the video to identify UI patterns, brand tokens, and navigation flows.
3. •Modernize: The platform generates a clean React component library and a corresponding E2E test suite.

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Why do traditional regression tools fail on legacy systems?#

Traditional automation tools like Selenium or early-stage Playwright scripts rely on stable DOM selectors (IDs, classes, or data-attributes). In legacy systems, these selectors are often autogenerated, inconsistent, or non-existent. If you cannot modify the source code to add

tags, your automation suite becomes brittle and prone to constant breakage.

Industry experts recommend moving away from selector-heavy testing in legacy environments. Instead, use a platform like Replay that uses visual context and temporal analysis. Because Replay understands the UI as a set of visual entities rather than just a DOM tree, it can detect regressions even when the underlying HTML structure shifts.

Comparison: Manual vs. Traditional vs. Replay#

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How do I automate E2E tests without touching the legacy repository?#

When you are building regression suite legacy infrastructure, your primary goal is to create a wrapper around the application. Replay’s Headless API allows AI agents (like Devin or OpenHands) to interact with your legacy app, record the output, and generate production-grade Playwright tests programmatically.

Here is an example of how you might trigger a regression test generation via a Replay webhook after a video recording is completed:

typescript
// Example: Triggering a test generation from a Replay recording
import { ReplayClient } from '@replay-build/sdk';

const replay = new ReplayClient(process.env.REPLAY_API_KEY);

async function generateLegacySuite(recordingId: string) {
  // Extracting the visual flow from the video
  const flow = await replay.extractFlow(recordingId);
  
  // Generating a Playwright test based on the video context
  const testScript = await replay.generateTest({
    format: 'playwright',
    browser: 'chromium',
    includeA11y: true
  });

  console.log("Generated Regression Test:", testScript);
  return testScript;
}

By using this approach, you are not just testing; you are documenting the system's "tribal knowledge" through visual recordings. Legacy Modernization Strategies often emphasize the need for this baseline before any refactoring begins.

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Can you generate React components from a legacy UI video?#

Yes. This is the core innovation of Replay. When you record a legacy application—whether it's built in Silverlight, Flash, or old-school ASP.NET—Replay’s engine performs Visual Reverse Engineering.

Visual Reverse Engineering is the methodology of reconstructing software architecture and UI components by analyzing the visual output and user interactions of a running application.

The platform doesn't just take a screenshot; it tracks the lifecycle of every pixel. It identifies what constitutes a "Button," a "Header," or a "Data Grid." It then maps these to your specific Design System. If you don't have a design system, Replay extracts brand tokens directly from the video to build one for you.

Example: Extracted Component Interface#

When Replay processes a legacy video, it outputs clean, modular React code that looks like this:

tsx
// Auto-extracted by Replay from Legacy CRM Video
import React from 'react';
import { Button, Input } from '@/components/ui';

interface LegacyInvoiceFormProps {
  onSubmit: (data: any) => void;
  initialData?: any;
}

export const LegacyInvoiceForm: React.FC<LegacyInvoiceFormProps> = ({ 
  onSubmit, 
  initialData 
}) => {
  return (
    <div className="p-6 bg-white shadow-md rounded-lg">
      <h2 className="text-xl font-bold mb-4">Invoice Details</h2>
      <form onSubmit={onSubmit} className="space-y-4">
        <Input 
          label="Client Name" 
          defaultValue={initialData?.client} 
          placeholder="Enter client name" 
        />
        <Button type="submit" variant="primary">
          Update Records
        </Button>
      </form>
    </div>
  );
};

This component is now decoupled from the legacy system. You have successfully moved from a "black box" to a modern React component that you can actually test and maintain.

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How to use Replay's Flow Map for navigation detection?#

One of the hardest parts of building regression suite legacy systems is mapping out the application's state machine. Legacy apps often have complex, hidden navigation logic. Replay’s Flow Map feature automatically detects multi-page navigation from the temporal context of your video.

If a user clicks "Settings," waits for a load screen, and then enters a "User Management" sub-menu, Replay identifies these as distinct states. It builds a visual map of the application, which serves as the blueprint for your regression suite. This ensures that your automated tests cover every possible branch of the user journey, not just the happy path.

For more on how this works, check out our guide on AI-Driven UI Extraction.

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Modernizing the "Un-modernizable"#

The global technical debt of $3.6 trillion exists because the cost of rewriting is seen as higher than the cost of maintaining. However, Replay flips this equation. By reducing the time to recreate a screen from 40 hours to 4 hours, you make modernization economically viable.

Replay is the only tool that generates component libraries from video, providing a bridge between the old world and the new. Whether you are moving from a mainframe-backed web portal to a modern Next.js stack or simply trying to ensure your legacy app doesn't break during a server migration, a video-first approach is the only way to capture 100% of the context.

Steps to Start Building Your Suite Today:#

1. •Identify Critical Paths: List the top 10 workflows that would "break the business" if they failed.
2. •Record with Replay: Use the Replay Chrome Extension to record these paths.
3. •Sync Design Tokens: Use the Replay Figma plugin to ensure the generated code matches your future-state brand.
4. •Generate Headless Tests: Use the Headless API to create a suite of Playwright tests that run against your legacy environment.
5. •Export Code: Take the generated React components and begin your incremental migration.

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Frequently Asked Questions#

Can I build a regression suite if the legacy app is behind a VPN?#

Yes. Replay offers On-Premise and SOC2/HIPAA-ready deployments. You can record and process videos within your secure environment without the data ever leaving your infrastructure. This is a standard requirement for regulated industries like banking and healthcare where legacy systems are most prevalent.

What happens if the legacy UI is inconsistent?#

Replay’s Agentic Editor uses surgical precision to handle UI inconsistencies. If a legacy app renders a button slightly differently across pages, the AI identifies these as variants of the same component rather than creating duplicate code. This deduplication is essential for building regression suite legacy assets that are actually maintainable.

Does Replay support mobile legacy applications?#

Replay is designed to work with any UI that can be recorded via video. By capturing the screen of a mobile emulator or a physical device, Replay can extract the same visual context and generate responsive React Native or web components and mobile-specific E2E tests.

How does Replay compare to simple screen recording tools?#

Simple screen recorders produce static video files. Replay produces Actionable Intelligence. While a video tells you what happened, Replay tells you how to recreate it in code. It extracts the design tokens, the component hierarchy, the state transitions, and the test logic, turning a passive recording into a production-ready asset.

Is the code generated by Replay actually production-ready?#

Yes. Unlike generic AI code generators that hallucinate logic, Replay bases its output on the exact visual evidence found in your recording. The generated code follows modern best practices, is fully typed with TypeScript, and is designed to be dropped into existing React projects.

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Ready to ship faster? Try Replay free — from video to production code in minutes.