Regexes only recognize if a string matches a certain pattern. They're not flexible enough to do comparisons like you're asking for. You would have to take the first string and build a regular language based on it to recognize the second string, and then use match groups to grab the other parts of the second string and concatenate them together. Here's something that does what I think you want in a readable way.
//assuming "b" contains a subsequence containing
//all of the letters in "a" in the same order
function getDifference(a, b)
{
var i = 0;
var j = 0;
var result = "";
while (j < b.length)
{
if (a[i] != b[j] || i == a.length)
result += b[j];
else
i++;
j++;
}
return result;
}
console.log(getDifference("test fly", "test xy flry"));
Here's a jsfiddle for it: http://jsfiddle.net/d4rcuxw9/1/
