Why my regex is not matching all the substrings?

Question

JS Code-

let N=7
let S="bab"


let arr=["a","b"]

for(i=2;i<N;i++){ //constructing a fibonnaci series
    let item=arr[i-1]+arr[i-2]
    arr.push(item)
}

// console.log(arr[N-1]) // babbababbabba

let marr=arr[N-1]

let str = new RegExp(S, "g");
let result=marr.match(str)

let answer=(result.length)
console.log(answer) // gives answer as 3 , but correct is 4

task at hand was to first construct a fibonnaci series where f1="a" and f2="b", For 3 onwards f3=f2+f1 and f4=f3+f2 Then we have to match the string S to see how many times it occurs in the fN. My code runs fine for rest of the test cases but for N=7 and S="bab" , correct answer is 4 but my code says 3.

See [https://stackoverflow.com/questions/20833295/how-can-i-match-overlapping-strings-with-regex](https://stackoverflow.com/questions/20833295/how-can-i-match-overlapping-strings-with-regex) — woocash19, Apr 27 '22 at 07:31
@RaayatAhmed no, the match starting from 3, ends at 5 (so you can't consider the match starting from 5). So the answer is 3: from index 0, 3 and 8. This is how regex works. If you want another result you can't use regex. — Giovanni Esposito, Apr 27 '22 at 08:34

score 0 · Answer 1 · answered Apr 27 '22 at 08:48

Inspired by this answer, you can't do this with a single regex, but you can do this:

let results = [];
let match;
let N=7
let pat = /(?=(bab))\w/g;
let arr=["a","b"]

for(i=2;i<N;i++){ //constructing a fibonnaci series
    let item=arr[i-1]+arr[i-2]
    arr.push(item)
}

// console.log(arr[N-1]) // babbababbabba

let marr=arr[N-1]

while ( (match = pat.exec( marr ) ) != null ) { 
  results.push( match[1] );
}

console.log(results.length);

You capture all three digits inside the lookahead, then go back and match one character in the normal way just to advance the match position.

Why my regex is not matching all the substrings?

1 Answers1