An example that uses lookarounds in Javascript to validate a password. Example from https://www.codewars.com/.

gistfile1.js

function validate(password) {
  return /^(?=.*\d)(?=.*[a-z])(?=.*[A-Z])[0-9a-zA-z]{6,}$/.test(password);
}

//If password contains at least one digit, one uppercase char, one lowercase char PLUS six or more 
//chars from the character class composed of one digit, one uppercase char, and one lowercase char.
> validate('djI38D55');
true

//passes if 6 or more chars
> validate('djI38D55666');
true

fails if no digits persent
> validate('djiDdF');
false

//fails if no lowercase chars present
> validate('DJI38D55');
false

// fails if no uppercase chars present
> validate('dji38d55');
false

//fails if less than 6 chars
> validate('djI38');
false