file:// URL paths can only be absolute:

file: URLs are always absolute paths.
— https://nodejs.org/api/fs.html#fs_file_url_paths

Yet the clean method strips out the CWD from the file: URLs, leaving the file:// at the start, making them invalid.

This is a pressing issue, because the error stack from errors thrown in an ESM file starts with the file: URL of the ESM. The ecosystem is currently migrating to vanilla ESM (in .mjs files, or .js files with package "type": "module") and more people will start to notice this bug.

In test.mjs:

import StackUtils from 'stack-utils';

const stackUtils = new StackUtils();

try {
  throw new Error('Message.');
} catch (error) {
  console.log(error.stack);
  console.log(stackUtils.clean(error.stack));
}

Then, run node test.mjs and the console log is ([CWD] substituting the absolute CWD path):

Error: Message.
    at file://[CWD]/test.mjs:6:9
    at ModuleJob.run (node:internal/modules/esm/module_job:175:25)
    at async Loader.import (node:internal/modules/esm/loader:178:24)
    at async Object.loadESM (node:internal/process/esm_loader:68:5)
file://test.mjs:6:9

Note the invalid file://test.mjs:6:9, it should be test.mjs:6:9.