Regex match from array of strings in JavaScript

Question

I am facing a bit problem in Regex. I want to find the start and end index of the complete matched string in the input string.

e.g. I have an array of strings like

["a", "aa"]

and I have a text like I like a problem aa

I am doing with iteration of array strings.

let arr = ["a", "aa"];
let str = "I like a problem aa";
let indicesArr = [];
arr.forEach(a=>{
  const regexObj = new RegExp(a, "gi");
  let match;
  while ((match = regexObj.exec(str))) {
    let obj = { start: match.index, end: regexObj.lastIndex }
    indicesArr.push(obj);
    if(!match.index || !regexObj.lastIndex) break;
  }
});

above code gives me the result

[
  {start: 7, end: 8},
  {start: 17, end: 18},
  {start: 18, end: 19},
  {start: 17, end: 19}
]

I want the result should be

[
  {start: 7, end: 8},
  {start: 17, end: 19}
]

Any suggestion would be very helpful, thanks:)

Use word boundaries to match a whole word, not a partial match. — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Aug 14, 2021 at 21:12
Hey, thank you so much for the suggestion,but if my string is I like a problem aa and aaaa, actually it's not matching the last aaaa part. — abinas patra
– abinas patra, Commented Aug 14, 2021 at 21:16
I am not getting, can you please write a small example, it would be helpful thanks:) — abinas patra
– abinas patra, Commented Aug 14, 2021 at 21:24

Wiktor Stribiżew · Accepted Answer · 2021-08-14 21:32:25Z

The problem here is that a finds two matches in aa. You need to make sure you match all occurrences of a regex that finds either aa or a in this order. It means, the regex must be /aa|a/g and not /a|aa/g as the order of alternation matters in regex.

Here, you can use

let arr = ["a", "aa"];
let str = "I like a problem aa";
let indicesArr = [];
arr.sort((a, b) => b.length - a.length);
const regexObj = new RegExp(arr.map(x=> x.replace(/[-\/\\^$*+?.()|[\]{}]/g, '\\$&')).join('|'), "gi");
let match;
while (match = regexObj.exec(str)) {
    let obj = { start: match.index, end: regexObj.lastIndex }
    indicesArr.push(obj);
}
console.log(indicesArr);

Note these two lines:

arr.sort((a, b) => b.length - a.length); - sorts the arr items by length in the descending order (to put aa before a)
const regexObj = new RegExp(arr.map(x=> x.replace(/[-\/\\^$*+?.()|[\]{}]/g, '\\$&')).join('|'), "gi"); - escapes all items in the arr array for use inside a regex, and joins the items with | alternation operator into a single string regex pattern.

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