Debug longest common substring

HTML

<div>

</div>

CSS

div { background-color: white; }

JavaScript

// Define strings
s1 = 'dadxx'
s2 = 'ddxx'

match = (a,b) => { 
let re, match, matches=[]
a.split('').forEach((l,i) => {
  re = new RegExp(l, 'g')
  while ((match = re.exec(b)) != null) {
   matches.push([i, match.index])
}
})
return matches
}

document.querySelector('div').innerHTML = JSON.stringify(match(s1,s2))