Regex to replace leading plus characters in query params (++, +- and +)?

I am using lunr.js.

This Javascript code below:

• gets the query parameters from the first search.
• replaces the binding + characters with spaces.
• stores the modified query in the search field for the next search.
• searches using the modified query.

var query = getUrlParameter('q');
var queryWithoutPlus = query.replace(/\+/g, " ");
searchField.value = queryWithoutPlus
index.search(queryWithoutPlus);

A query param string could look like test+ABCD+Test++Test2+-Test+-Test+Test.

The code above replaces the + characters in the query parameter q with spaces to display the result to the user in a nice manner,

Currently, a search string spaces

"test +test -test"

currently results in

"test test -test"

What I need is:

"test +test -test"

I tried to modify the resulting query multiple times using a tempQuery:

var tempQuery = query.replace(/\+\+/g, " -");
var queryWithoutPlus = tempQuery.replace(/\+\-/g, " -");

But this doesn't work out with the remaining + characters and doesn't feel correct.

Does it just boil down to using the correct regex (whatever it might be, advice welcome), or is there is even a better approach for using query parameters with lunr.js?

You need a negative lookbehind query (?<!\+)

"test+ABCD+Test++Test2+-Test+-Test+Test".replace(/(?<!\+)\+/g, " ")
>> "test ABCD Test +Test2 -Test -Test Test"

As noted, this is not available on some browsers, like Microsoft's. In that case you need to replace with a temporary expression

"test+ABCD+Test++Test2+-Test+-Test+Test".replace(/\+\+/g, "#spaceplus#")
  .replace(/\+/g, " ").replace(/#spaceplus#/g, " +")
>> "test ABCD Test +Test2 -Test -Test Test"