Regex searching for recurring substring with overlap search

Question

"Your goal is to find the longest recurring substring in the input (ignoring case) and returning a lower-case version of that string."

I was solving a little problem, and wanted to use regex, but the expected output overlaps.

My Code:

let x = "Is this thing on?"
console.log((x.match(/(.+)(?=.*\1)/gi)||[]).sort((a,b)=>b.length-a.length)[0].toLowerCase())

Expected answer: "is thi"

My answer: "is th"

Is it possible to solve this problem using regex? If so, how?

Answer 1

The problem here is that the repeated chunk start position occurs before the end of the Group 1 match.

The only thing you can do with regex is capture all overlapping matches of any text that is immediately preceded with this very text that is immediately followed with any one or more text and then this very text again:

/(?=(.*)(?<=(?=.+\1)\1))./sgi

See the regex demo.

Sample implementation in JavaScript:

let x = "Is this thing on?"
console.log(
  (Array.from(x.matchAll(/(?=(.*)(?<=(?=.+\1)\1))/gsi), x=>x[1])||[])
    .sort((a,b)=>b.length-a.length)[0]
    .toLowerCase()
)