regex to find specific strings in javascript

The g modifier does not affect exec, because exec only returns the first match by specification. What you want is the match method:

mystring.match(/[a-z,-]+=/g)

No regex necessary. Write a tiny parser, it's easy.

function parseValues(str) {
    var result = {};

    str.split("||").forEach(function (item) {
        var parts = item.split("=");
        result[ parts[0] /* key */ ] = parts[1]; /* value */
    });

    return result;
}

usage

var obj = parseValues("subject=something||x-access-token=something-else");
// -> {subject: "something", x-access-token: "something-else"}

var subj = obj.subject;
// -> "something"

var token = obj["x-access-token"];
// -> "something-else"

Additional complications my arise when there is an escaping schema involved that allows you to have || inside a value, or when a value can contain an =.

You will hit these complications with regex approach as well, but with a parser-based approach they will be much easier to solve.

You have to execute exec twice to get 2 extracted strings.

According to MDN: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/exec

If your regular expression uses the "g" flag, you can use the exec() method multiple times to find successive matches in the same string.

Usually, people extract all strings matching the pattern one by one with a while loop. Please execute following code in browser console to see how it works.

var regex = /[a-z,-]+=/g;
var string = "subject=something||x-access-token=something";
while(matched = regex.exec(string)) console.log(matched);

You can convert the string into a valid JSON string, then parse it to retrieve an object containing the expected data.

var str = 'subject=something||x-access-token=something';
var obj = JSON.parse('{"' + str.replace(/=/g, '":"').replace(/\|\|/g, '","') + '"}');
console.log(obj);

I don't think you need regexp here, just use the javascript builtin function "split".

  var s = "subject=something1||x-access-token=something2";
  var r = s.split('||'); // r now is an array: ["subject=something1", "x-access-token=something2"]
  var i;
  for(i=0; i<r.length; i++){
    // for each array's item, split again
    r[i] = r[i].split('=');
  }

At the end you have a matrix like the following:

   y     x 0                1
   0       subject          something1
   1       x-access-token   something2

And you can access the elements using x and y:

"subject" == r[0][0]
"x-access-token" == r[1][0]
"something2" == r[1][1]

If you really want to do it with a pure regexp:

var input = 'subject=something1||x-access-token=something2'
var m = /subject=(.*)\|\|x-access-token=(.*)/.exec(input)
var subject = m[1]
var xAccessToken = m[2]
console.log(subject);
console.log(xAccessToken);

However, it would probably be cleaner to split it instead:

console.log('subject=something||x-access-token=something'
      .split(/\|\|/)
      .map(function(a) { 
        a = a.split(/=/); 
        return { key: a[0], val: a[1] }
      }));