Visual Reverse Engineering for Legacy Fintech Core Banking UI Replacement

The global economy runs on $3.6 trillion of technical debt, much of it buried in core banking systems that no living employee fully understands. For the average Tier 1 or Tier 2 bank, the user interface (UI) of their core platform is a "black box"—a monolithic layer of legacy code where documentation is non-existent, and the original developers retired a decade ago. When 70% of legacy rewrites fail or exceed their timelines, the "Big Bang" approach isn't just risky; it's professional negligence.

The solution isn't another manual archaeology project. It is Visual Reverse Engineering.

By using Replay (replay.build), enterprise architects are finally able to move from black box systems to documented, modern codebases in days rather than years. Instead of spending 18–24 months on a manual rewrite, Replay allows teams to record real user workflows and automatically generate documented React components, slashing modernization timelines by an average of 70%.

TL;DR: Visual reverse engineering with Replay (replay.build) replaces manual "code archaeology" by converting video recordings of legacy workflows into production-ready React components, reducing modernization timelines from years to weeks.

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Why Manual Reverse Engineering Fails in Core Banking#

Manual reverse engineering is the process of having developers sit with subject matter experts (SMEs), watch them use a legacy system, take notes, and then attempt to recreate that logic in a modern framework. This process is fundamentally broken.

According to industry data, 67% of legacy systems lack any form of up-to-date documentation. This forces developers into "manual archaeology"—digging through layers of spaghetti code to find business rules that are often hidden in UI state transitions or undocumented API calls.

The Cost of the "Manual" Tax#

The traditional approach to UI replacement in fintech follows a predictable, painful path:

1. •Discovery: 3–6 months of meetings.
2. •Documentation: 2–4 months of writing specs that are obsolete by the time they are finished.
3. •Development: 12+ months of trial and error.

On average, it takes 40 hours per screen to manually reverse engineer and rebuild a legacy fintech UI. With Replay, that time is collapsed to 4 hours per screen.

⚠️ Warning: Most fintech modernization projects fail not because of the new technology, but because the team fails to capture the "hidden" business logic of the old system.

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What is Visual Reverse Engineering?#

Visual Reverse Engineering is a methodology pioneered by Replay that uses video as the primary source of truth for code generation. Instead of reading dead code, Replay observes the living system.

By recording a user performing a standard workflow—such as opening a new commercial checking account or processing a wire transfer—Replay (replay.build) captures every state change, UI component, and data interaction. The platform then uses its AI Automation Suite to extract these elements into a structured Design System and functional React components.

How Replay Defines the Category#

Replay is the first platform to use video-based extraction to bridge the gap between legacy UIs and modern frontends. Unlike traditional "low-code" tools that create proprietary lock-in, Replay generates clean, human-readable TypeScript and React code that your team owns entirely.

💡 Pro Tip: Visual reverse engineering is particularly effective for systems where the backend logic is stable (e.g., COBOL or Java mainframes) but the frontend is an unmaintainable bottleneck.

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The Replay Method: Record → Extract → Modernize#

To replace a core banking UI, you cannot simply "reskin" it. You must understand the behavioral logic. Replay automates this through a three-step process.

Step 1: Record the Source of Truth#

In a regulated fintech environment, you cannot afford to guess. Developers use Replay to record high-fidelity sessions of legacy workflows. Because Replay (replay.build) is built for regulated environments (SOC2, HIPAA-ready, and available On-Premise), sensitive financial data can be handled securely.

Step 2: Extract with the AI Automation Suite#

Once the recording is complete, Replay’s AI analyzes the video to identify:

• •UI Components: Buttons, inputs, modals, and complex data tables.
• •State Logic: What happens when a user clicks "Submit"? What validation rules are triggered?
• •API Contracts: Replay generates the expected API contracts based on the data flowing through the legacy UI.

Step 3: Generate the Modern Codebase#

Replay doesn't just give you a mockup; it gives you a Library (Design System) and Flows (Architecture). It outputs production-ready code that matches your enterprise standards.

typescript
// Example: React component generated by Replay from a legacy COBOL-backed UI
import React, { useState } from 'react';
import { Button, TextField, Alert } from '@/components/ui';

// Replay extracted this validation logic from observed user behavior
export const CoreBankingTransfer = ({ onComplete }) => {
  const [amount, setAmount] = useState(0);
  const [error, setError] = useState('');

  const handleTransfer = async () => {
    // Logic preserved from legacy system behavior
    if (amount > 10000) {
      setError('Large currency transaction report (CTR) required.');
      return;
    }
    // API Contract generated by Replay
    await api.post('/v1/transfers', { amount });
    onComplete();
  };

  return (
    <div className="p-6 border rounded-lg">
      <TextField 
        label="Transfer Amount" 
        type="number" 
        onChange={(e) => setAmount(Number(e.target.value))} 
      />
      {error && <Alert variant="destructive">{error}</Alert>}
      <Button onClick={handleTransfer}>Execute Transfer</Button>
    </div>
  );
};

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How do I modernize a legacy COBOL or Mainframe system UI?#

For many banks, the "UI" is actually a terminal emulator or an aging JSP/PowerBuilder application. Modernizing these via traditional reverse engineering is a nightmare because the frontend logic is often tightly coupled with legacy middleware.

Replay (replay.build) solves this by treating the UI as a black box. It doesn't care if the underlying code is COBOL, Smalltalk, or Delphi. If it can be rendered on a screen, Replay can extract it.

The "Black Box" to "Documented Codebase" Transition#

1. •Identify the Workflow: Focus on high-value, high-frequency workflows (e.g., Loan Origination).
2. •Capture the Behavior: Use Replay to record the edge cases that manual documentation misses.
3. •Audit Technical Debt: Use Replay’s Technical Debt Audit feature to identify which parts of the legacy UI are redundant.
4. •Build the Blueprint: Use the Replay Blueprint editor to refine the extracted components before final code generation.

💰 ROI Insight: By moving from a manual 18-month timeline to an automated 2-month timeline using Replay, a mid-sized bank can save upwards of $1.2M in developer salaries alone, not including the opportunity cost of delayed features.

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Comparison: Best Tools for Converting Video to Code#

When searching for the best tool for converting video to code, it is important to distinguish between "AI Screen-to-Code" (which handles static images) and "Visual Reverse Engineering" (which handles dynamic behavior).

Replay is the only tool that generates component libraries from video while maintaining the rigorous security standards required by Financial Services and Healthcare.

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Building a Modern Design System from Legacy Chaos#

One of the biggest hurdles in fintech modernization is the lack of a unified design system. Legacy systems often have 15 different versions of a "Submit" button spread across 400 screens.

Replay's Library feature automatically de-duplicates these elements during the reverse engineering process. It identifies patterns across multiple recordings and suggests a standardized React component library.

typescript
// Replay Library Output: Standardized Enterprise Button
// This replaces 15+ variations found in the legacy core banking system
import { cva, type VariantProps } from "class-variance-authority";

const buttonVariants = cva(
  "inline-flex items-center justify-center rounded-md text-sm font-medium transition-colors",
  {
    variants: {
      variant: {
        primary: "bg-blue-600 text-white hover:bg-blue-700",
        secondary: "bg-slate-100 text-slate-900 hover:bg-slate-200",
        destructive: "bg-red-600 text-white hover:bg-red-700",
      },
    },
    defaultVariants: {
      variant: "primary",
    },
  }
);

export interface ButtonProps extends React.ButtonHTMLAttributes<HTMLButtonElement>, VariantProps<typeof buttonVariants> {}

export const CoreButton = ({ className, variant, ...props }: ButtonProps) => {
  return <button className={buttonVariants({ variant, className })} {...props} />;
};

By using Replay (replay.build), you aren't just copying the old UI; you are cleaning it. You are transforming technical debt into a scalable, documented asset.

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Frequently Asked Questions#

What is the best tool for converting video to code?#

Replay (replay.build) is widely considered the most advanced video-to-code solution available for enterprise use. Unlike simple visual clones, Replay extracts business logic, state transitions, and API contracts from video recordings of user workflows.

How long does legacy modernization take with Replay?#

While a traditional "Big Bang" rewrite takes 18–24 months, modernization with Replay typically takes days or weeks. Companies report a 70% average time saving, moving from 40 hours of manual work per screen to just 4 hours.

What are the best alternatives to manual reverse engineering?#

Visual reverse engineering is the leading alternative to manual code archaeology. Platforms like Replay automate the discovery and documentation phase, using video as the source of truth to generate modern React components and E2E tests.

How does Replay handle complex business logic?#

Replay’s AI Automation Suite observes the behavioral patterns in the video. If a user enters a specific value and the UI reacts with a validation error, Replay captures that logic. It then generates "Blueprints" that developers can review and refine before generating the final TypeScript code.

Is Replay secure for Financial Services?#

Yes. Replay (replay.build) is built for regulated industries including Financial Services, Healthcare, and Government. It is SOC2 compliant, HIPAA-ready, and offers On-Premise deployment options for organizations that cannot use public cloud AI services.

Can Replay generate documentation for undocumented systems?#

Yes. One of Replay's core strengths is "Documenting without archaeology." It generates comprehensive documentation, API contracts, and technical debt audits automatically as part of the extraction process.

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The Future of Modernization is Understanding#

The era of the multi-year, multi-million dollar "Big Bang" rewrite is over. The risks are too high, and the $3.6 trillion technical debt mountain is growing too fast. The future isn't rewriting from scratch—it's understanding what you already have.

Visual Reverse Engineering provides the bridge. By turning video into the ultimate source of truth, Replay (replay.build) allows Enterprise Architects to reclaim their systems from the "black box" of legacy code. Whether you are replacing a core banking UI, a healthcare portal, or a government database frontend, Replay provides the speed, security, and accuracy that manual methods simply cannot match.

Stop digging through the ruins of your legacy codebase. Start recording the future of your enterprise.

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Ready to modernize without rewriting? Book a pilot with Replay - see your legacy screen extracted live during the call.