Using regexp to find variable group of letters in a string

Question

I'm doing a basic algorithm scripting challenge on FCC and I want to return true if a string in a first element of an array contains all of the letters of the string in the second element of the array, or false otherwise.

I've written some code for this. However I can't seem to pass one test:

mutation(["hello", "Hello"])

I've tried removing the global flag and have tried using constructor notation and literal notation based on recommendations from the FCC community, but to no avail.

This is the code:

function mutation(arr) {
  let patt = new RegExp("[arr.1]", "i");
  return patt.test(arr[0]);

}

mutation(["hello", "Hello"])

The function is supposed to return true instead it returns false. What is wrong with my code?

What do you want to match with `[arr.1]` pattern? It matches a single char, `a`, `r`, `.` or `1`. Nothing close to `hello` or `Hello`. — Wiktor Stribiżew, Aug 06 '19 at 08:35
You don't need a regex for this: `[...second].every(x => first.includes(x))` — georg, Aug 06 '19 at 08:36

T.J. Crowder · Accepted Answer · 2019-08-06T09:10:12.887

new RegExp("[arr.1]", "i") uses [arr.1] (literally) as the regular expression, which is why it doesn't work.

I wouldn't use a regular expression for this, it's simpler to do it directly. For instance:

function mutation(arr) {
  const lc = arr[0].toLowerCase();
  return [...arr[1].toLowerCase()].every(ch => lc.includes(ch));
}

...or to make searching the string not be a linear search:

function mutation(arr) {
  const chars = new Set([...arr[0].toLowerCase()]);
  return [...arr[1].toLowerCase()].every(ch => chars.has(ch));
}

...but if you have to use regex, you could generate an array with all the permutations of arr[1] (["Hello", "oHell", "loHel", ...]) (perhaps using the answers here) then create your regular expression using beginning/ending anchors (^, $) and alternations (|) of all the permutations:

let permutations = /*...use the linked answers to create permutations...*/;
let patt = new RegExp("^(?:" + permutations.join("|") + ")$", "i");

If arr[1] may contain characters that are special in regular expressions, you'll need to escape them (perhaps using one of the answers here):

let patt = new RegExp("^(?:" + permutations.map(escapeRegExp).join("|") + ")$", "i");

(Again: There is no built-in escapeRegExp, see the answers to the question linked above for how to create one.)

Thanks for pointing that out. (For some reason FCC lets it pass most of the tests, which is why I didnt catch that error). But when I used your code, it passes the test even if it only matches with one letter. I want it to match all the letters. — raeign, Aug 06 '19 at 08:46
@raeign - Doh! Of course it does. :-) (The RegExp version. The others are right.) I wouldn't use RegExp for this, I'd use one of the other solutions. But hmmmm..... — T.J. Crowder, Aug 06 '19 at 08:57
wow, i see now why no one wants to use regex for this question. Thanks for your help! — raeign, Aug 06 '19 at 09:08

Using regexp to find variable group of letters in a string

1 Answers1