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Luca Liu
Luca Liu

Posted on • Originally published at blog.luca-liu.com

Markdown Code Blocks Breaking HTML Rendering: How to Fix Line Break Issues

#markdown #html #webdev

Markdown Code Blocks Breaking HTML Rendering: How to Fix Line Break Issues

The Problem: Code Blocks Losing Line Breaks

When building a custom markdown renderer, one of the most frustrating issues is code blocks that break HTML rendering. Your code suddenly loses all line breaks and gets wrapped in the wrong HTML tags.

Here's what happens:

Before (Broken):

<pre class="language-python">
  <code>import requestsimport pandas as pdimport xml.etree.ElementTree as ET</code>
</pre>
<p class="text-gray-700">
  <code>root = ET.fromstring(login_response.text)</code>
</p>
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After (Fixed):

<pre class="language-python">
  <code>import requests
import pandas as pd
import xml.etree.ElementTree as ET

root = ET.fromstring(login_response.text)</code>
</pre>
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Why This Happens: The Root Cause

The problem occurs in the markdown processing order. Here's the sequence that breaks everything:

1. Wrong Processing Order 

// ❌ WRONG: This breaks code blocks
function markdownToHtml(markdown: string): string {
  // Step 1: Replace code blocks with placeholders
  html = html.replace(codeBlockRegex, (match, language, code) => {
    const placeholder = `__CODE_BLOCK_${index}__`
    return placeholder
  })

  // Step 2: Restore code blocks immediately
  codeBlockPlaceholders.forEach((placeholder, index) => {
    const codeBlockHtml = `<pre><code>${code}</code></pre>`
    html = html.replace(placeholder, codeBlockHtml)
  })

  // Step 3: Process paragraphs (THIS BREAKS EVERYTHING!)
  html = html.replace(/^(.*)$/gim, '<p class="text-gray-700">$1</p>')
}
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The Problem: After restoring code blocks, the paragraph processing runs and wraps everything in <p> tags, including your already-rendered code blocks.

2. The Critical Line That Breaks Everything 

// ❌ THIS LINE IS THE CULPRIT
html = html.replace(/^(.*)$/gim, '<p class="text-gray-700">$1</p>')
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This regex pattern matches every line and wraps it in <p> tags, even lines that are already inside <pre> tags.

The Solution: Protected Placeholder System

The fix is to protect code blocks from paragraph processing using a double-layer placeholder system.

Step 1: Detect Code Blocks First 

// ✅ RIGHT: Detect code blocks and replace with placeholders
const codeBlockRegex = /```
{% endraw %}
([a-zA-Z0-9_-]*)[\s]*\n([\s\S]*?)\n
{% raw %}
```/g
const codeBlockPlaceholders = []

html = html.replace(codeBlockRegex, (match, language, code) => {
  const placeholder = `__CODE_BLOCK_${codeBlockPlaceholders.length}__`
  codeBlockPlaceholders.push({ match, language, code })
  return placeholder
})
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Step 2: Protect Placeholders from Paragraph Processing 

// ✅ RIGHT: Convert code block placeholders to protected placeholders
const protectedPlaceholders = []
html = html.replace(/(__CODE_BLOCK_\d+__)/g, (match) => {
  const placeholder = `__PROTECTED_${protectedPlaceholders.length}__`
  protectedPlaceholders.push({ placeholder, content: match })
  return placeholder
})
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Step 3: Process Paragraphs (Safely) 

// ✅ RIGHT: Now process paragraphs - protected placeholders are safe
html = html
  .replace(/\n\n/g, '</p><p class="text-gray-700">')
  .replace(/\n/g, '<br />')
  .replace(/^(.*)$/gim, '<p class="text-gray-700">$1</p>')
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Step 4: Restore Everything in Correct Order 

// ✅ RIGHT: Restore protected placeholders first
protectedPlaceholders.forEach(({ placeholder, content }) => {
  html = html.replace(placeholder, content)
})

// ✅ RIGHT: Finally restore code blocks AFTER all processing
codeBlockPlaceholders.forEach((placeholder, index) => {
  const { language, code } = placeholder
  const codeBlockHtml = `<pre class="language-${language}"><code>${code}</code></pre>`
  const placeholderText = `__CODE_BLOCK_${index}__`
  html = html.replace(placeholderText, codeBlockHtml)
})
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The Key CSS Fix

Even with the correct processing order, you need proper CSS to preserve line breaks:

/* ✅ RIGHT: This preserves line breaks */
.prose-pre {
  white-space: pre !important;  /* Critical for line breaks */
  word-wrap: normal !important;
  overflow-x: auto !important;
}

.prose-pre code {
  white-space: pre !important;  /* Also critical */
  background: transparent !important;
  font-family: monospace !important;
}
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Why This Solution Works

1. Processing Order Protection

  • Code blocks are detected first
  • Placeholders are protected during paragraph processing
  • Code blocks are restored last

2. Double-Layer Protection

  • __CODE_BLOCK_X__ → __PROTECTED_X__ → Final HTML
  • Each layer prevents interference from other processing steps

3. CSS Whitespace Preservation

  • white-space: pre ensures line breaks are respected
  • Monospace font maintains code formatting

Common Mistakes to Avoid

❌ Don't Process Code Blocks Early 

// Wrong: Code blocks get processed before paragraphs
codeBlockPlaceholders.forEach(/* restore code blocks */)
html = html.replace(/^(.*)$/gim, '<p>$1</p>') // This breaks code blocks!
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❌ Don't Use Optional Newlines in Regex 

// Wrong: Optional newline can cause parsing issues
const codeBlockRegex = /```
{% endraw %}
(\w+)?\n?([\s\S]*?)
{% raw %}
```/g

// Right: Force newline after language
const codeBlockRegex = /```
{% endraw %}
([a-zA-Z0-9_-]*)[\s]*\n([\s\S]*?)\n
{% raw %}
```/g
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❌ Don't Forget CSS Whitespace 

/* Wrong: Missing whitespace preservation */
.prose pre { @apply bg-gray-900; }

/* Right: Explicit whitespace preservation */
.prose-pre { white-space: pre !important; }
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Testing Your Fix

After implementing the solution, test with complex code blocks:

def test_function():
    # This should have proper line breaks
    result = some_complex_operation(
        param1="value1",
        param2="value2"
    )
    return result
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Check the generated HTML - it should look like:

<pre class="language-python">
  <code>def test_function():
    # This should have proper line breaks
    result = some_complex_operation(
        param1="value1",
        param2="value2"
    )
    return result</code>
</pre>
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Summary

The key to fixing markdown code block line break issues is:

  1. Process code blocks first - Detect and replace with placeholders
  2. Protect placeholders - Use double-layer protection system
  3. Process other elements - Headers, paragraphs, links, etc.
  4. Restore code blocks last - After all other processing is complete
  5. Use proper CSS - white-space: pre is essential

This approach ensures that code blocks are completely isolated from paragraph processing and maintain their formatting integrity.

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Luca Liu

Hello there! 👋 I'm Luca, a Business Intelligence Developer with passion for all things data. Proficient in Python, SQL, Power BI, Tableau, SAP Business Objects.

Thank you for taking the time to explore data-related insights with me. I appreciate your engagement.

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