Hoisted from the comments

2020 comment: rather than using regex, we now have URLSearchParams, which does all of this for us, so no custom code, let alone regex, are necessary anymore.

– Mike 'Pomax' Kamermans

Browser support is listed here https://caniuse.com/#feat=urlsearchparams

I would suggest an alternative regex, using sub-groups to capture name and value of the parameters individually and re.exec():

function getUrlParams(url) {
  var re = /(?:\?|&(?:amp;)?)([^=&#]+)(?:=?([^&#]*))/g,
      match, params = {},
      decode = function (s) {return decodeURIComponent(s.replace(/\+/g, " "));};

  if (typeof url == "undefined") url = document.location.href;

  while (match = re.exec(url)) {
    params[decode(match[1])] = decode(match[2]);
  }
  return params;
}

var result = getUrlParams("http://maps.google.de/maps?f=q&source=s_q&hl=de&geocode=&q=Frankfurt+am+Main&sll=50.106047,8.679886&sspn=0.370369,0.833588&ie=UTF8&ll=50.116616,8.680573&spn=0.35972,0.833588&z=11&iwloc=addr");

result is an object:

{
  f: "q"
  geocode: ""
  hl: "de"
  ie: "UTF8"
  iwloc: "addr"
  ll: "50.116616,8.680573"
  q: "Frankfurt am Main"
  sll: "50.106047,8.679886"
  source: "s_q"
  spn: "0.35972,0.833588"
  sspn: "0.370369,0.833588"
  z: "11"
}

The regex breaks down as follows:

(?:            # non-capturing group
  \?|&         #   "?" or "&"
  (?:amp;)?    #   (allow "&", for wrongly HTML-encoded URLs)
)              # end non-capturing group
(              # group 1
  [^=&#]+      #   any character except "=", "&" or "#"; at least once
)              # end group 1 - this will be the parameter's name
(?:            # non-capturing group
  =?           #   an "=", optional
  (            #   group 2
    [^&#]*     #     any character except "&" or "#"; any number of times
  )            #   end group 2 - this will be the parameter's value
)              # end non-capturing group

See this question:

txt = "Local residents o1__have called g__in o22__with reports...";
var regex = /o([0-9]+)__/g
var matches = [];
var match = regex.exec(txt);
while (match != null) {
    matches.push(match[1]);
    match = regex.exec(txt);
}
alert(matches);

The thing you need to understand about regular expressions is that multiple runs of the matcher will only find non-overlapping targets. If your regex is capturing too much, then you will not be able to find extra matches with additional runs.

Try this regular expression, which captures less:

([^;]+?)\s+\(([^\)]*)\)

It has two capturing groups, which grab the interesting prefix and the other interesting stuff in the brackets. Please note that you will need to use String.trim() on the results. Here is the regex explained on Regex 101.

Here is the final JavaScript solution, which includes the regex:

var complexString = 'some boring stuff; some other boring stuff; interesting prefix (interesting inner stuff1; interesting inner stuff2; etc.); boring stuff; another interesting prefix (another interesting string 1; another interesting string 2; etc.)';
var roundBraketsRegex =  /([^;]+?)\s+\(([^\)]*)\)/g;        
var matchesArray;
var i = 1;

while (matchesArray = roundBraketsRegex.exec(complexString)) {
    var group1 = matchesArray[1].trim();
    var group2 = matchesArray[2].trim();
    console.log("Match #" + i + " [1]: '" + group1 + "' [2]: '" + group2 + "'");
    ++i;
}

Here is the output from running the above:

Match #1 [1]: 'interesting prefix' [2]: 'interesting inner stuff1; interesting inner stuff2; etc.'
Match #2 [1]: 'another interesting prefix' [2]: 'another interesting string 1; another interesting string 2; etc.'

I hope that you find this helpful.

--Jonathan

exec() is returning only the set of captures for the first match, not the set of matches as you expect. So what you're really seeing is $0 (the entire match, "a") and $1 (the first capture)--i.e. an array of length 2. exec() meanwhile is designed so that you can call it again to get the captures for the next match. From MDN:

If your regular expression uses the "g" flag, you can use the exec method multiple times to find successive matches in the same string. When you do so, the search starts at the substring of str specified by the regular expression's lastIndex property (test will also advance the lastIndex property).

You have to use the g flag for this to work on multiple occurrences, and you need to match only words (\w), the dot (\.) and slash (\/) before the dot and the extension:

let re = /([\w\.\/]*)\.(?:jpg|png)/g

console.log(
    'Hello ../aaa.jpg ../bbb.jpg ../sss.gif ../xxx.png End of Text'.match(re)
)