Today I Learned #3: Javascript RegExp gotcha

Whenever I find myself working with strings I usually end up using regex to life easier. I was quite shocked to stumble on a regex gotcha within Javascript while working on Find the unique string.

A quick summary of the problem. You have a list of strings made using the same characters except one string made up of unique characters. My solution takes the first string, converts it into a regular expression and then splits the strings based on if they match the other strings. Finally I return the first string from the shortest list.

RegEx solution

function findUniq(arr) {
  // do magic
  let pick = new Set(arr[0]);
  let reg = new RegExp(`[^${[...pick].join("")}]`, "ig");
  /// assuming its generic
  let withPick = [];
  let without = [];
  for (let i = 0; i < arr.length; i++) {
    let current = arr[i];
    if (reg.test(current)) {
    } else {

  if (without.length == 1) {
    return without[0];
  } else {
    return withPick[0];
let t = ["Tom Marvolo Riddle", "I am Lord Voldemort", "Harry Potter"]; // 'Harry Potter'

It seemed to pass the basic tests but the result wasn’t reliable. It would pass one test and the same test on retries. The reason was that I had included the global ‘g’ modifier and if you reuse the same same regex with different strings, it will move to the lastIndex Of the last match. In a sense it is treating each new string you are testing as a continuation of the last matched word. To avoid that situation I decided to remove the global modifier. I could have also initialized the regEx for each line I was testing.


End of story