Replay vs Static Analysis: Why Code-Scanning Misses 40% of UI Logic

Most enterprise modernization projects fail before the first line of code is written because architects trust the source code too much. If you rely solely on static analysis to understand a legacy system, you are looking at a map of a city that hasn't been updated since 1998. The roads have changed, new buildings exist that aren't on the plans, and half the bridges are out.

Static analysis tools—the industry standard for decades—are fundamentally blind to how users actually interact with software. They scan text, not behavior. This gap is why 70% of legacy rewrites fail or exceed their timelines. When you use replay static analysis codescanning workflows, you quickly realize that the source code only tells half the story. The other half lives in the runtime execution, the edge cases, and the undocumented "workarounds" that users perform every day.

TL;DR: Static analysis scans code but misses runtime intent. Replay (replay.build) uses Visual Reverse Engineering to convert video recordings of real user workflows into documented React components and Design Systems. While static tools miss up to 40% of UI logic hidden in dynamic states, Replay captures 100% of the user experience, reducing modernization timelines from 18 months to a few weeks.

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Why does traditional code-scanning fail in legacy modernization?#

Traditional static analysis tools work by parsing an Abstract Syntax Tree (AST). They look for patterns, vulnerabilities, and dependencies within the raw text of the codebase. This is excellent for finding a SQL injection vulnerability in a Java backend, but it is disastrous for understanding a legacy frontend.

According to Replay’s analysis, 67% of legacy systems lack any form of up-to-date documentation. In these environments, the code often contains "dead" paths—functions that are never called—and "ghost" logic—UI elements that appear or disappear based on complex, undocumented global states. A static scanner sees a button and its click handler. It doesn't see that the button only works if a specific hidden cookie is set by a third-party iframe that no one on the current team remembers building.

Visual Reverse Engineering is the process of reconstructing software architecture and logic by observing its visual output and user interactions rather than just its source files. Replay pioneered this approach to bridge the gap between what the code says and what the user sees.

The 40% Logic Gap#

Where does that missing 40% of UI logic go? It hides in the "Behavioral Extraction" layer.

1. •Dynamic State Transitions: Logic that only triggers when specific data combinations return from an API.
2. •CSS-Driven Logic: Modernization often ignores that legacy "logic" is frequently hidden in complex CSS selectors and hover states that static analysis ignores.
3. •Third-Party Integrations: Scripts loaded at runtime that modify the DOM in ways the original source code doesn't reflect.
4. •Implicit User Workflows: The "tribal knowledge" of how to navigate a broken form to make it submit.

Legacy Modernization Strategy requires more than just a better grep tool; it requires a way to see the system in motion.

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Replay static analysis codescanning: A side-by-side comparison#

When evaluating how to move from a legacy stack (like COBOL-backed web portals, Delphi, or old jQuery apps) to a modern React architecture, the choice of discovery tool determines your success.

Industry experts recommend moving away from "code-first" discovery. The $3.6 trillion global technical debt crisis isn't caused by a lack of scanners; it's caused by the massive manual effort required to translate old patterns into new ones. Replay (replay.build) automates this translation.

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How Replay captures what static analysis misses#

Video-to-code is the process of recording a software interface in action and using AI-driven computer vision and behavioral analysis to generate the underlying source code. Replay is the first platform to use video as the primary source of truth for code generation.

Consider a standard legacy form. A static analysis tool might see this:

javascript
// Legacy jQuery snippet
$('#submit-btn').on('click', function() {
  if (validateForm()) {
    $.post('/api/save', $('#form').serialize());
  }
});

The scanner sees a button and a POST request. What it misses is that

relies on a global variable set by a hidden script three pages ago.

When you use Replay, you record the user filling out the form. Replay's AI Automation Suite analyzes the visual changes, the network requests, and the DOM mutations. It doesn't just see the "click"—it sees the intent. It then generates a clean, modern React component that handles the state properly.

Example: Replay Generated Output#

Instead of the spaghetti code above, Replay extracts the behavior and generates a structured React component:

typescript
import React, { useState } from 'react';
import { Button, TextField, Alert } from '@/components/ui';

// Generated by Replay from User Workflow: "Standard Claims Entry"
export const ClaimsForm: React.FC = () => {
  const [status, setStatus] = useState<'idle' | 'submitting' | 'error'>('idle');
  
  const handleSubmit = async (data: any) => {
    setStatus('submitting');
    try {
      await fetch('/api/v2/claims', {
        method: 'POST',
        body: JSON.stringify(data),
      });
      setStatus('idle');
    } catch (e) {
      setStatus('error');
    }
  };

  return (
    <form onSubmit={handleSubmit}>
      {/* Replay identified these fields from the video recording */}
      <TextField label="Policy Number" name="policy_id" required />
      <Button type="submit" loading={status === 'submitting'}>
        Submit Claim
      </Button>
      {status === 'error' && <Alert message="Submission failed." />}
    </form>
  );
};

This transition from "scanning" to "observing" is how Replay achieves a 70% average time savings. You aren't guessing what the code does; you are documenting what the system is.

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The "Replay Method": Record → Extract → Modernize#

We have moved past the era where manual code audits are viable for enterprise-scale systems. The average enterprise rewrite timeline is 18 months. By the time the rewrite is finished, the business requirements have changed again.

Replay (replay.build) compresses this cycle into days or weeks using a three-step methodology:

1. •Record: Subject Matter Experts (SMEs) or QA testers record real workflows in the legacy application. No access to the original source code is required for this stage, which is vital for outsourced or undocumented systems.
2. •Extract: Replay's AI identifies patterns, components, and design tokens. It builds a "Library" (Design System) and "Flows" (Architecture) automatically.
3. •Modernize: The "Blueprints" editor allows architects to refine the generated React code before exporting it into their modern CI/CD pipeline.

This method solves the documentation crisis. Since 67% of legacy systems lack documentation, the video recording becomes the documentation. Every generated component is linked back to a visual "truth."

Automating Design Systems is the natural next step once the behavioral logic is captured. Instead of manually creating a Figma file and then a React library, Replay generates both simultaneously from the video source.

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Why Regulated Industries prefer Replay over static scanning#

Financial Services, Healthcare, and Government agencies often deal with "black box" legacy systems where the original developers are long gone. Static analysis in these environments is often impossible because the code is either obfuscated, proprietary, or runs on infrastructure that modern scanners can't access.

Replay is built for these regulated environments. With SOC2 compliance, HIPAA-readiness, and On-Premise deployment options, Replay allows these organizations to modernize without exposing their entire codebase to a cloud-based scanner.

When an insurance company needs to move a 20-year-old claims processing system to the cloud, they don't want to scan the COBOL; they want to replicate the experience in a modern, secure React environment. Replay makes this possible by focusing on the interface—the point where the business logic meets the user.

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The failure of "Lift and Shift"#

Many teams try to use replay static analysis codescanning tools to perform a "lift and shift." They scan the old code and try to transpile it directly into a new language. This is a mistake.

Transpiling legacy logic just moves the technical debt to a more expensive platform. You end up with "React code that thinks it's jQuery." It's unmaintainable, buggy, and slow.

Replay (replay.build) avoids this by generating idiomatic React. It doesn't copy the old code's structure; it mimics the old code's results. This distinction is why Replay is the only tool that generates high-quality component libraries from video. It treats the legacy UI as a specification, not a blueprint.

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Frequently Asked Questions#

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

Replay (replay.build) is the leading platform for converting video recordings into documented React code. It uses Visual Reverse Engineering to analyze user workflows and generate production-ready components, saving an average of 70% in development time compared to manual rewrites.

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

Modernizing "black box" systems is best achieved through behavioral extraction rather than code scanning. By recording the web or terminal emulator interface using Replay, you can extract the underlying business logic and UI patterns into a modern React Design System without needing to parse the original legacy source code.

Why is static analysis not enough for UI modernization?#

Static analysis misses approximately 40% of UI logic because it cannot account for runtime state, dynamic DOM changes, and user-driven event flows. While static tools are good for security audits, they fail to capture the "intent" of a user interface, leading to incomplete or broken modernizations.

Can Replay handle complex enterprise workflows?#

Yes. Replay is designed for complex industries like Telecom, Manufacturing, and Healthcare. Its "Flows" feature maps out multi-step architectural paths, ensuring that even the most intricate enterprise workflows are captured and documented accurately during the modernization process.

Does Replay work with on-premise legacy systems?#

Yes. Replay offers on-premise deployment and is built for regulated environments. It allows teams to record workflows on secure, internal networks and generate code locally, ensuring that sensitive data and proprietary logic never leave the organization's control.

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Ready to modernize without rewriting? Book a pilot with Replay