JSFiddle - React, Tailwind, and code Playground

HTML

<p id="output">The match was found on line </p>

JavaScript

var string = "This\nstring\nhas\nmultiple\nlines.";
  astring = string.split('\n');
  console.log(astring);
  match = /has/;

Array.each(astring, function(line, number) {
  if (match.exec(line))
    $('output').innerHTML += number + 1;
});