Terser: Only mangle matched property names from regex

I want to keep ALL my variables and function names, even the private ones, and only mangles the ones that match the regex.

The docs say:

regex (default: null) — Pass a RegExp literal or pattern string to only mangle property matching the regular expression.

But this will also mangles all private const in function scope like so: (keep_fnames and keep_classnames are set to true).

Config:

const terserOptions = {
  compress: {
    defaults: false,
  },
  format: {
    beautify: true,
    comments: "all",
  },
  mangle: {
    keep_fnames: true,
    properties: {
      regex: /^_/,
    },
  },
};

Input:

function myFunc(arg) {
  const keepThisVarName = arg * 2;
  return {
    keepThisProp: keepThisVarName,
    _mangleThisOne: keepThisVarName,
  }
}

Output:

function myFunc(e) {
  const t = 2 * e;
  return {
    keepThisProp: t,
    p: t
  }
}

Expected output:

function myFunc(arg) {
  const keepThisVarName = arg * 2;
  return {
    keepThisProp: keepThisVarName,
    p: keepThisVarName, // '_mangleThisOne' should be mangled
  }
}

Am I missing something or is it simply not possible to prevent Terser to mangle everything he can by default? If not, any ideas how to mangle only variables that match a regular expression and keep the rest of my codebase intact?

Thanks!

Looking at the source, code it doesn't seem possible to prevent mangling private methods when using the mangle option. Terser will always run mangle_private_properties() when mangle is defined.

if (options.mangle) {
    toplevel.compute_char_frequency(options.mangle);
    toplevel.mangle_names(options.mangle);
    toplevel = mangle_private_properties(toplevel, options.mangle);
}

https://github.com/terser/terser/blob/a47f29a57b12999c939845781daa7d6f8bdaeff2/lib/minify.js#L264