How to Fix: [v15] Module parse failed: Bad character escape sequence

The Module parse failed: Bad character escape sequence error in Next.js v15 usually appears before your app even renders, and it almost always points to one thing: a string, regex, import path, or generated source file contains an invalid escape sequence that the current parser refuses to accept.

In the reported reproduction using a fresh app created with create-next-app, the failure is typically triggered by how the project is being parsed by the Next.js v15 build pipeline, especially when special characters, Windows-style path escapes, or invalid escaped characters make their way into source code or generated files.

Understanding the Root Cause

This error happens when the JavaScript or TypeScript parser encounters a backslash-based escape that is not valid in the current context.

Common examples include:

• Using Windows-style backslashes inside JavaScript strings without escaping them properly.
• Accidentally writing invalid sequences like \m, \_, or malformed Unicode escapes.
• Embedding file system paths directly into source code.
• A dependency or generated file being compiled with syntax that the active loader rejects.
• Using a regex or string copied from another source where slashes were altered.

Why this shows up in Next.js 15 more visibly than before: the newer toolchain is stricter about parsing and surfaces syntax issues earlier. If a file contains an illegal escape sequence, the bundler fails during module parsing instead of allowing the error to hide until runtime.

A classic example is this:

const filePath = 'C:\new\test'

This is valid because each backslash is escaped. But this version is broken:

const filePath = 'C:
ew	est'

In that broken string, \n becomes a newline escape, and other path fragments may become invalid or unintended escapes. The parser then throws the Bad character escape sequence error.

The same principle applies to route definitions, regex literals, environment-derived strings, and code generated by tools during project creation.

Step-by-Step Solution

Use the following process to identify and fix the failing module.

1. Reproduce the error with a clean install

rm -rf .next node_modules package-lock.json yarn.lock pnpm-lock.yaml
npm install
npm run dev

If you are on Windows PowerShell, remove folders using the appropriate shell commands, then reinstall dependencies and run the dev server again.

2. Read the exact file path in the stack trace

The error output usually includes the module that failed to parse. Check whether the failing file is:

• Your own source file
• A generated file under .next
• A dependency inside node_modules

If it points to your app code, inspect recent edits first. If it points to generated output, clear caches and regenerate.

3. Search for invalid backslash usage

Inspect strings, regex literals, config files, and path definitions. Typical fixes include converting backslashes to either escaped backslashes or forward slashes.

Broken:

const assetPath = 'C:	emp
ew-folderile.js'

Fixed:

const assetPath = 'C:\temp\new-folder\file.js'

Also valid:

const assetPath = 'C:/temp/new-folder/file.js'

4. Check next.config.js, custom loaders, and environment-driven code

If the issue appears immediately after project creation or dependency changes, inspect project configuration files for invalid string escapes.

/** @type {import('next').NextConfig} */
const nextConfig = {
  reactStrictMode: true,
}

module.exports = nextConfig

If your config contains path aliases, copied regex patterns, or manually constructed strings, sanitize them.

5. Validate regex literals

Invalid regex escaping can trigger similar parser failures.

Broken:

const pattern = /some	ext/

Safer alternatives:

const pattern = /some\\text/
const pattern2 = new RegExp('some\\text')

Make sure the intended escaping matches the syntax context: regex literal versus string passed to RegExp.

6. Remove and recreate the app if the scaffold itself is corrupted

If a brand-new project fails right after running npx create-next-app@latest, regenerate it with the latest stable toolchain:

npx create-next-app@latest my-app
cd my-app
npm run dev

If the issue persists, also update Node.js to a supported version and retry.

7. Verify your Node.js version

Next.js 15 expects a modern Node runtime. An unsupported version can surface misleading parser or dependency issues.

node -v

Then compare it against the current Next.js support matrix in the official Next.js documentation.

8. Reinstall using one package manager only

Mixed lockfiles can pull inconsistent dependency trees.

rm -rf node_modules package-lock.json yarn.lock pnpm-lock.yaml
npm install
npm run dev

Use only one of npm, yarn, or pnpm per project.

9. If the failure comes from a dependency, pin or update it

Sometimes the invalid escape is inside a third-party package or generated code consumed by the bundler. In that case:

• Update the dependency to the latest version.
• Downgrade to the last known working version if this is a regression.
• Check the package issue tracker.
• Test the app with a minimal dependency set.

npm update
npm run dev

10. Compare against the reproduction sandbox

If you are working from the original report, inspect the linked CodeSandbox reproduction and compare:

• Node version
• Next.js version
• Generated project files
• Any custom edits added after scaffolding

This often reveals whether the issue is environmental or caused by a specific file mutation.

Common Edge Cases

• Windows paths inside source files: A path copied from Explorer can silently introduce invalid escapes.
• Environment variables: A value from .env may be injected into code and parsed unexpectedly if manually transformed.
• Regex copied from another language: Escape rules differ between JavaScript, shell commands, and config formats.
• Generated code from plugins: A loader, codegen step, or custom script may output malformed JavaScript.
• Mismatched dependency tree: Lockfile drift can install parser-related packages that behave differently across environments.
• Editor auto-formatting or paste transformations: Some editors convert slashes or quotes in ways that break syntax.

If the file path in the error points to generated code, treat the generated file as a symptom, not the source. The real bug is often in config, a plugin, or a dependency that produced that file.

FAQ

Why does this happen in a brand-new Next.js 15 app?

If a fresh scaffold fails, the cause is usually environmental: unsupported Node.js, corrupted dependency installation, a package manager mismatch, or a regression in a generated dependency path. Recreating the app, cleaning lockfiles, and updating Node are the fastest checks.

Is this a Next.js bug or a JavaScript syntax bug?

Usually it is a syntax parsing issue surfaced by the Next.js bundler. Next.js is not necessarily generating bad syntax itself; it is often exposing invalid escapes in app code, config, or dependencies earlier in the build process.

What is the safest way to avoid escape sequence issues?

Prefer forward slashes in portable paths when possible, escape backslashes correctly in JavaScript strings, validate regex patterns carefully, and avoid manually embedding raw system paths into source code.

In short, fix the issue by locating the exact parsed module, removing invalid escape sequences, cleaning the install, and validating your runtime and dependency versions. In most Next.js v15 cases, the error is not random—it is a strict parser correctly rejecting malformed escaped characters somewhere in the module graph.