// ten characters, followed by the first character again, followed by the user input
let getMatch = input => new RegExp(`^(.)(.)(.)(.)(.)(.)(.)(.)(.)(.)\\1${escape(input)}$`);

// seems to work!
console.log(getMatch('x').test('abcdefghijax')); // true

// unless the user input starts with a 0...
console.log(getMatch('0').test('abcdefghija0')); // false
console.log(getMatch('0').test('abcdefghijj')); // true