Line Ending Parsing

Line Ending Parsing

Category: Cross-Platform Time Saved: 30 minutes debugging string comparisons Battle-tested: Yes — breaks pattern matching silently

The Problem

You split a text file into lines and process each one. It works on macOS/Linux. On Windows, your regex patterns fail to match, or string comparisons return false even though the content looks identical.

Why It Happens

Windows uses \r\n (CRLF) for line endings. Unix uses \n (LF).

// File content on Windows:
"line1\r\nline2\r\nline3"

// After split("\n"):
["line1\r", "line2\r", "line3"]
//      ↑ invisible \r still attached!

// Your pattern fails:
"line1\r" === "line1"  // false
/^line1$/.test("line1\r")  // false

The Rule

Always use /\r?\n/ regex for splitting text files into lines.

// ❌ WRONG — leaves \r on Windows
const lines = content.split('\n');

// ✅ CORRECT — handles both CRLF and LF
const lines = content.split(/\r?\n/);

Implementation Pattern

const fs = require('fs');

function parseTextFile(filepath) {
  const content = fs.readFileSync(filepath, 'utf8');
  
  // Normalize line endings
  const lines = content.split(/\r?\n/);
  
  // Or use a helper
  return normalizeLines(content);
}

function normalizeLines(text) {
  // Option 1: Split with regex
  return text.split(/\r?\n/);
  
  // Option 2: Normalize then split
  // return text.replace(/\r\n/g, '\n').split('\n');
}

When This Matters

Common Symptoms

• "Pattern doesn't match on Windows"
• "String comparison fails but they look the same"
• "Works on Mac, breaks on Windows"
• console.log shows correct content but code doesn't match

Debugging Hidden Characters

// Reveal hidden characters
function showHidden(str) {
  return str
    .replace(/\r/g, '\\r')
    .replace(/\n/g, '\\n');
}

console.log(showHidden(line));
// "expected value\r" ← the culprit!

Related Scenarios

Trimming Doesn't Always Help

// trim() removes \r, but only at ends
"  line1\r  ".trim()  // "line1\r" — \r still there if spaces follow!

// Be explicit
line.replace(/\r$/, '').trim()

Reading Streams

// readline module handles this automatically
const rl = readline.createInterface({ input: stream });
rl.on('line', (line) => {
  // line has no \r or \n
});

Writing Cross-Platform

const os = require('os');

// Write with platform-appropriate line endings
const output = lines.join(os.EOL);

// Or force Unix line endings (common in config files)
const output = lines.join('\n');

Verification Checklist

• All split('\n') calls use /\r?\n/ instead
• String comparisons work on Windows test machine
• Config/CSV parsing tested with CRLF files
• Git autocrlf settings considered for repos

Related Skills

• cloud-storage-paths — Cross-platform file access
• terminal-backtick-hazard — Cross-platform command execution