Insure that regex moves to the second OR element only if the first one doesn't exist

Question

I'm trying to match a certain word on a string and only if it doesn't exist i want to match the another one using the OR | operator ....but the match is ignoring that... how can i insure that the behavior works :

const str = 'Soraka is an ambulance 911'
const regex = RegExp('('+'911'+'|'+'soraka'+')','i')
console.log(str.match(regex)[0])     // should get 911 instead

Answer 1

911 occurs late in the string, whereas Soraka occurs earlier, and the regex engine iterates character-by-character, so Soraka gets matched first, even though it's on the right-hand side of the alternation.

One option would be to match Soraka or 911 in captured lookaheads instead, and then with the regex match object, alternate between the two groups to get the one which is not undefined:

const check = (str) => {
  const regex = /^(?=.*(911)|.*(Soraka))/;
  const match = str.match(regex);
  console.log(match[1] || match[2]);
};

check('Soraka is an ambulance 911');
check('foo 911');
check('foo Soraka');