hideEmailName.ts

const hide = (s: string) => '*'.repeat(s.length);
const hideEachSecondMatch = (_match: string, ...matches: string[]) =>
  matches
    .slice(0, -2)
    .map((m, i) => (i % 2 ? hide(m) : m))
    .join('');
// Hide all characters in email name, except first and last character
// FIXME: Better hide at least one character for 1-2 character names
const hideEmail = (email: string, options?: { domain?: boolean }) =>
  email.replace(
    options?.domain ? /^(.?)(.*)(.@.)(.*)(\.[^.]+)$/ : /^(.)(.*)(.@.+)/,
    hideEachSecondMatch
  );

hideEmail('[email protected]', { domain: true }); // 'e@o******.com'
hideEmail('[email protected]', { domain: true }); // 'em@o******.com'
hideEmail('[email protected]', { domain: true }); // 'e*a@o******.com'
hideEmail('[email protected]', { domain: true }); // 'e***l@o******.com'
hideEmail('[email protected]'); // 'e***[email protected]'

No words, great solution regexp beast ❤️