Get URL parameter with regular expression

Summary:
With this RegEx you can extract the value of a URL GET parameter.
Note: This code is used to explain the concept. It works perfectly fine. A refined code respecting URL encoded values is at the bottom of page.

function getQueryParam(param) {
  var rx = new RegExp("[?&]" + param + "=([^&]+).*$");
  var returnVal = window.location.search.match(rx);
  return returnVal === null ? "" : returnVal[1];
}

Explanation of the code

Function wrap

function getQueryParam(param) {
        // ...
}

Obviously we want to be able to extract any parameter. We can call this function for every param that we want to extract its value from the URL. Like so:

// wanting to get the parameter "myParam":
getQueryParam("myParam");

Declaration of RegEx

var rx = new RegExp("[?&]" + param + "=([^&]+).*$");

1. We are using "new RegExp" to define the regular expression. This gives us the possibility to dynamically insert the "param" every time. We do so by concatenating the strings with the "+".
2. We need to make sure, the param we are looking for follows directly after either a "?" or a "&". Otherwise, if looking for "pants" would match "&pants=on" as well as "&underpants=on (we only want to match the former). We do this by looking for the characters "?" and "&" by means of "[?&]"
3. We use the group "([^&]+)" to match one ore more characters not being "&" to get our return value. This way this group will contain all characters that are not "&". It will match up to the next "&" or the end of the URL query string. The "+" quantifier causes this group to only be a match if there is one or more characters in this group.
4. Because we quantify this group "([^&]+)" with the "+" the entire RegEx will only match if this group holds at least one character. So a query like "?pants=&otherParam=value" will cause the entire RegEx to not match and return an empty string "" for the param "pants" (which is correct). Same for this query string: "?pants&otherParam=val".
5. By having the "=" before this group we make sure that our group follows directly after the "=".
6. Then finally we essentially tell the RegEx that we don't care what comes after this second group by matching ".*$". We could omit this and our RegEx would still work fine since we are not extracting any of this match. However, I always like formulating RegExes to their expected end to have clearer speaking code.

Returning the value

var match = window.location.search.match(rx);
return returnVal === null ? "" : returnVal[1];

We apply the regular expression to the URL query string by means of the .match() method. The second line checks if the RegEx did match at all (if not returnVal will be "null").

If we have no match we return an empty string. If we have a match then we return that.

Refined version of code

function getQueryParam(param) {
    var rx = new RegExp("[?&]" + param + "=([^&]+).*$");
    var returnVal = window.location.search.match(rx);
    return returnVal === null ? "" : decodeURIComponent(returnVal[1].replace(/\+/g, " "));
}

This is an extension of the above code. Besides the description above this code also accounts for encoded URL parameter values: In case the value of the param we are looking for has been URL encoded we decode it before returning. Spaces decoded as "+" are changed into spaces again.

Further reading

get cookie value (very similar case)
Different ways of declaring RegExes
Escaping in regular expressions