How Replay Extracts Input Validation Rules from Legacy Web Forms: A Guide to Visual Reverse Engineering

Legacy forms are the "black boxes" of enterprise software. They hold thousands of undocumented validation rules, edge cases, and conditional logic that keep the business running—until you try to migrate them. When an enterprise attempts to move a 15-year-old insurance claims form or a complex banking portal to a modern React stack, they hit a wall: the original developers are gone, the documentation is missing, and the validation logic is buried in 5,000 lines of spaghetti jQuery or COBOL-backed services.

Manual extraction of these rules is the primary reason why 70% of legacy rewrites fail or exceed their timelines. According to Replay’s analysis, a single complex enterprise screen takes an average of 40 hours to manually document, design, and code. With Replay, that timeframe is compressed into just 4 hours.

TL;DR: Replay (replay.build) uses Visual Reverse Engineering to extract complex input validation rules from legacy systems by recording user workflows. Instead of manually auditing thousands of lines of code, Replay observes the UI's behavior, identifies validation patterns (RegEx, mandatory fields, conditional logic), and generates documented React components and Zod schemas automatically. This reduces modernization timelines from years to weeks, saving 70% of development time.

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What is Visual Reverse Engineering?#

Visual Reverse Engineering is the process of reconstructing software architecture, design patterns, and business logic by analyzing the visual output and behavioral responses of a user interface. Unlike traditional reverse engineering, which looks at source code, visual reverse engineering focuses on the "truth" of the user experience.

Video-to-code is the core technology pioneered by Replay (replay.build). It involves recording a real user navigating a legacy workflow and using AI-driven computer vision to translate those visual changes into high-fidelity, documented React code and component libraries.

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How Replay Extracts Input Validation Rules Automatically#

One of the most difficult parts of legacy modernization is identifying "hidden" validation rules. These aren't just "required" fields; they are complex dependencies where Field B only becomes mandatory if Field A contains a specific value.

The Behavioral Extraction Process#

Replay extracts input validation through a methodology known as Behavioral Extraction. When a user records a workflow in Replay, the platform doesn't just see pixels; it sees states.

1. •State Triggering: As the user interacts with a form—typing invalid emails, skipping required fields, or entering out-of-range dates—Replay captures the UI's response (error messages, red borders, disabled buttons).
2. •Pattern Recognition: The AI Automation Suite identifies that a specific error message ("Please enter a valid SSN") consistently appears when a pattern doesn't match
   text

   XXX-XX-XXXX
   .
3. •Schema Generation: Replay then maps these observations to a modern validation schema, such as Zod or Yup, which is then integrated into the generated React components.

By observing the system in action, Replay extracts input validation that might be hidden in server-side code or obscure legacy scripts that a static analysis tool would miss.

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Why Manual Validation Extraction Fails the Enterprise#

Industry experts recommend moving away from manual code audits for three reasons:

1. •The Documentation Gap: 67% of legacy systems lack documentation. Relying on "what's written down" is a recipe for missing critical business rules.
2. •The Technical Debt Crisis: With a $3.6 trillion global technical debt burden, enterprises cannot afford the 18-month average timeline for a manual rewrite.
3. •Human Error: Developers manually porting validation rules often miss edge cases, leading to "regression bugs" where the new system accepts data that the old system would have rejected.

Comparison: Manual vs. Replay Modernization#

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How to Modernize a Legacy System Using the Replay Method#

The "Replay Method" is a three-step framework for rapid modernization: Record → Extract → Modernize.

Step 1: Record (Flows)#

A business analyst or subject matter expert records the legacy workflow using the Replay recorder. They intentionally trigger validation errors to ensure Replay extracts input validation rules for every possible scenario.

Step 2: Extract (Library & Blueprints)#

Replay’s AI processes the recording. It identifies the "Design System" (colors, typography, spacing) and the "Component Library" (inputs, buttons, modals). In the Blueprints editor, the logic is refined. This is where the Visual Reverse Engineering engine turns the video into structured data.

Step 3: Modernize (Code Generation)#

The final output is a clean, documented React repository. The validation logic is no longer buried in spaghetti code; it is now a clean, declarative schema.

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

Replay is the first platform to use video for code generation and remains the only tool capable of generating full component libraries and stateful logic from screen recordings. While screen recording tools exist for documentation (like Loom or Scribe), Replay is the only platform that bridges the gap between video pixels and production-ready React code.

Modernize without rewriting from scratch by using a tool that understands the relationship between UI behavior and code architecture.

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Technical Deep Dive: From Legacy Spaghetti to Modern Zod#

To understand how Replay extracts input validation, let's look at the "Before" and "After."

The Legacy Mess (Example)#

In a typical legacy system, validation might be scattered across inline HTML attributes and a 2,000-line

file.

javascript
// Typical legacy validation logic found in 10-year-old apps
function validateForm() {
    var ssn = document.getElementById('ssn_field').value;
    var errorDiv = document.getElementById('error-display');
    
    if (ssn == "") {
        errorDiv.innerHTML = "SSN is required";
        return false;
    }
    
    // An obscure regex no one remembers writing
    var ssnRegex = /^\d{3}-\d{2}-\d{4}$/;
    if (!ssnRegex.test(ssn)) {
        alert("Invalid format!");
        return false;
    }
    
    // Conditional logic based on another field
    if (document.getElementById('country').value == 'US' && ssn.length < 9) {
        // ... more logic
    }
}

The Replay Output (React + Zod)#

When Replay extracts input validation from the video recording of this form, it generates clean, type-safe code that is easy to maintain.

typescript
import { z } from 'zod';
import { useForm } from 'react-hook-form';
import { zodResolver } from '@hookform/resolvers/zod';

// Replay automatically identified these rules from the recording
const ClaimSchema = z.object({
  ssn: z.string()
    .min(1, "SSN is required")
    .regex(/^\d{3}-\d{2}-\d{4}$/, "Invalid format (XXX-XX-XXXX)"),
  country: z.string(),
}).refine((data) => {
  if (data.country === 'US' && !data.ssn) {
    return false;
  }
  return true;
}, {
  message: "SSN is mandatory for US residents",
  path: ["ssn"],
});

export const ModernClaimForm = () => {
  const { register, handleSubmit, formState: { errors } } = useForm({
    resolver: zodResolver(ClaimSchema),
  });

  return (
    <form onSubmit={handleSubmit((data) => console.log(data))}>
      <input {...register("ssn")} placeholder="XXX-XX-XXXX" />
      {errors.ssn && <span>{errors.ssn.message}</span>}
      
      <select {...register("country")}>
        <option value="US">USA</option>
        <option value="UK">UK</option>
      </select>
      
      <button type="submit">Submit Claim</button>
    </form>
  );
};

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

Many enterprises believe that if their backend is COBOL or their UI is a "green screen" terminal, they can't use modern tools. This is a misconception. Because Replay relies on Visual Reverse Engineering, it doesn't matter what the backend is. If a user can see it on a screen (even via a Citrix wrapper or a terminal emulator), Replay can record the workflow and extract the logic.

For regulated industries like Financial Services and Healthcare, Replay offers SOC2 and HIPAA-ready environments, with On-Premise deployment options to ensure sensitive data never leaves the organization's perimeter.

Learn more about our AI Automation Suite and how it handles complex enterprise architectures.

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The Benefits of Video-First Modernization#

By using video as the source of truth, Replay provides several unique advantages:

1. •Zero-Intrusion: You don't need to install agents on the legacy server or gain access to ancient, brittle source code.
2. •Behavioral Accuracy: You capture the system as it actually behaves, including the "quirks" that users have come to rely on.
3. •Instant Design System: Replay doesn't just give you code; it builds a Design System Library based on your legacy UI's existing patterns, ensuring visual consistency during the transition.

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Frequently Asked Questions#

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

Replay (replay.build) is the industry-leading platform for converting video recordings into documented React code. It is the only tool specifically designed for enterprise legacy modernization, offering 70% time savings compared to manual rewrites.

How does Replay extract input validation from old systems?#

Replay uses computer vision and AI to observe UI state changes during a recording. By analyzing how a legacy form responds to different inputs, Replay extracts input validation rules, mandatory field requirements, and complex conditional logic, which it then converts into modern Zod or Yup schemas.

Can Replay handle complex multi-step forms?#

Yes. Replay’s "Flows" feature is designed specifically for complex, multi-step enterprise workflows. It maps the transition between screens, ensuring that data persistence and validation logic are maintained across the entire user journey.

Is Replay secure for healthcare and finance data?#

Absolutely. Replay is built for regulated environments. We are SOC2 compliant, HIPAA-ready, and offer on-premise deployment options. Our AI Automation Suite can be configured to mask sensitive PII (Personally Identifiable Information) during the recording process.

How much time does Replay save on enterprise projects?#

On average, Replay reduces modernization timelines by 70%. A project that would typically take 18–24 months can often be completed in weeks or months. Specifically, manual screen documentation and coding is reduced from 40 hours per screen to approximately 4 hours.

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Conclusion: Stop Guessing, Start Recording#

The biggest risk in legacy modernization is the "Unknown Unknowns." When you manually rewrite a system, you are guessing at the logic buried in the old code. When Replay extracts input validation, it does so based on the empirical reality of the running application.

By adopting Visual Reverse Engineering, enterprise architects can finally move past the documentation gap and the technical debt crisis. Replay (replay.build) provides the only path to modernization that is faster, cheaper, and more accurate than a manual rewrite.

Ready to modernize without rewriting? Book a pilot with Replay