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. code blocks that break HTML rendering Here's what happens: Before (Broken): Before (Broken): <pre class="language-python">
 <code>import requestsimport pandas as pdimport xml.etree.ElementTree as ET</code>
</pre>

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

 <code>root = ET.fromstring(login_response.text)</code>
 After (Fixed): 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> <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> Why This Happens: The Root Cause The problem occurs in the markdown processing order. Here's the sequence that breaks everything: markdown processing order 1. Wrong Processing Order 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, '$1')
} // ❌ WRONG: This breaks code blocks
function markdownToHtml(markdown: string): string {
 // Step 1: Replace code blocks with placeholders
 html = html.replace(codeBlockRegex, (match, language, code) => {
 const placeholder = `__CODE_BLOCK_${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, '$1')
} The Problem: After restoring code blocks, the paragraph processing runs and wraps everything in tags, including your already-rendered code blocks. The Problem: 2. The Critical Line That Breaks Everything The Critical Line That Breaks Everything // ❌ THIS LINE IS THE CULPRIT
html = html.replace(/^(.*)$/gim, '$1') // ❌ THIS LINE IS THE CULPRIT
html = html.replace(/^(.*)$/gim, '$1') This regex pattern matches every line and wraps it in tags, even lines that are already inside <pre> tags. every line <pre> The Solution: Protected Placeholder System The fix is to protect code blocks from paragraph processing using a double-layer placeholder system. protect code blocks Step 1: Detect Code Blocks First Step 1: Detect Code Blocks First // ✅ RIGHT: Detect code blocks and replace with placeholders
const codeBlockRegex = /```([a-zA-Z0-9_-]*)[\s]*\n([\s\S]*?)\n```/g
const codeBlockPlaceholders = []

html = html.replace(codeBlockRegex, (match, language, code) => {
 const placeholder = `__CODE_BLOCK_${codeBlockPlaceholders.length}__`
 codeBlockPlaceholders.push({ match, language, code })
 return placeholder
}) // ✅ RIGHT: Detect code blocks and replace with placeholders
const codeBlockRegex = /```([a-zA-Z0-9_-]*)[\s]*\n([\s\S]*?)\n```/g
const codeBlockPlaceholders = []

html = html.replace(codeBlockRegex, (match, language, code) => {
 const placeholder = `__CODE_BLOCK_${codeBlockPlaceholders.length}__`
 codeBlockPlaceholders.push({ match, language, code })
 return placeholder
}) Step 2: Protect Placeholders from Paragraph Processing Step 2: Protect Placeholders from Paragraph Processing // ✅ RIGHT: Convert code block placeholders to protected placeholders
const protectedPlaceholders = []
html = html.replace(/(__CODE_BLOCK_\d+__)/g, (match) => {
 const placeholder = `__PROTECTED_${protectedPlaceholders.length}__`
 protectedPlaceholders.push({ placeholder, content: match })
 return placeholder
}) // ✅ RIGHT: Convert code block placeholders to protected placeholders
const protectedPlaceholders = []
html = html.replace(/(__CODE_BLOCK_\d+__)/g, (match) => {
 const placeholder = `__PROTECTED_${protectedPlaceholders.length}__`
 protectedPlaceholders.push({ placeholder, content: match })
 return placeholder
}) Step 3: Process Paragraphs (Safely) Step 3: Process Paragraphs (Safely) // ✅ RIGHT: Now process paragraphs - protected placeholders are safe
html = html
 .replace(/\n\n/g, '')
 .replace(/\n/g, ' ')
 .replace(/^(.*)$/gim, '$1') // ✅ RIGHT: Now process paragraphs - protected placeholders are safe
html = html
 .replace(/\n\n/g, '')
 .replace(/\n/g, ' ')
 .replace(/^(.*)$/gim, '$1') Step 4: Restore Everything in Correct Order 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)
}) // ✅ 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)
}) 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;
} /* ✅ 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;
} Why This Solution Works 1. Processing Order Protection Processing Order Protection Code blocks are detected first
Placeholders are protected during paragraph processing
Code blocks are restored last Code blocks are detected first Placeholders are protected during paragraph processing Code blocks are restored last 2. Double-Layer Protection Double-Layer Protection __CODE_BLOCK_X__ → __PROTECTED_X__ → Final HTML
Each layer prevents interference from other processing steps __CODE_BLOCK_X__ → __PROTECTED_X__ → Final HTML __CODE_BLOCK_X__ __PROTECTED_X__ Each layer prevents interference from other processing steps 3. CSS Whitespace Preservation CSS Whitespace Preservation white-space: pre ensures line breaks are respected
Monospace font maintains code formatting white-space: pre ensures line breaks are respected white-space: pre Monospace font maintains code formatting Common Mistakes to Avoid ❌ Don't Process Code Blocks Early Don't Process Code Blocks Early // Wrong: Code blocks get processed before paragraphs
codeBlockPlaceholders.forEach(/* restore code blocks */)
html = html.replace(/^(.*)$/gim, '$1') // This breaks code blocks! // Wrong: Code blocks get processed before paragraphs
codeBlockPlaceholders.forEach(/* restore code blocks */)
html = html.replace(/^(.*)$/gim, '$1') // This breaks code blocks! ❌ Don't Use Optional Newlines in Regex Don't Use Optional Newlines in Regex // Wrong: Optional newline can cause parsing issues
const codeBlockRegex = /```(\w+)?\n?([\s\S]*?)```/g

// Right: Force newline after language
const codeBlockRegex = /```([a-zA-Z0-9_-]*)[\s]*\n([\s\S]*?)\n```/g // Wrong: Optional newline can cause parsing issues
const codeBlockRegex = /```(\w+)?\n?([\s\S]*?)```/g

// Right: Force newline after language
const codeBlockRegex = /```([a-zA-Z0-9_-]*)[\s]*\n([\s\S]*?)\n```/g ❌ Don't Forget CSS Whitespace 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; } /* Wrong: Missing whitespace preservation */
.prose pre { @apply bg-gray-900; }

/* Right: Explicit whitespace preservation */
.prose-pre { white-space: pre !important; } 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 def test_function():
 # This should have proper line breaks
 result = some_complex_operation(
 param1="value1",
 param2="value2"
 )
 return result 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> <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> Summary The key to fixing markdown code block line break issues is: Process code blocks first - Detect and replace with placeholders
Protect placeholders - Use a double-layer protection system
Process other elements - Headers, paragraphs, links, etc.
Restore code blocks last - After all other processing is complete
Use proper CSS - white-space: pre is essential Process code blocks first - Detect and replace with placeholders Process code blocks first Protect placeholders - Use a double-layer protection system Protect placeholders Process other elements - Headers, paragraphs, links, etc. Process other elements Restore code blocks last - After all other processing is complete Restore code blocks last Use proper CSS - white-space: pre is essential Use proper CSS white-space: pre This approach ensures that code blocks are completely isolated from paragraph processing and maintain their formatting integrity.

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