Regular expression to remove one parameter from query string

Question

I'm looking for a regular expression to remove a single parameter from a query string, and I want to do it in a single regular expression if possible.

Say I want to remove the foo parameter. Right now I use this:

/&?foo\=[^&]+/ 

That works as long as foo is not the first parameter in the query string. If it is, then my new query string starts with an ampersand. (For example, "foo=123&bar=456" gives a result of "&bar=456".) Right now, I'm just checking after the regex if the query string starts with ampersand, and chopping it off if it does.

Example edge cases:

Input                    |  Expected Output -------------------------+-------------------- foo=123                  |  (empty string) foo=123&bar=456          |  bar=456 bar=456&foo=123          |  bar=456 abc=789&foo=123&bar=456  |  abc=789&bar=456 

---

Edit

OK as pointed out in comments there are there are way more edge cases than I originally considered. I got the following regex to work with all of them:

/&foo(\=[^&]*)?(?=&|$)|^foo(\=[^&]*)?(&|$)/ 

This is modified from Mark Byers's answer, which is why I'm accepting that one, but Roger Pate's input helped a lot too.

Here is the full suite of test cases I'm using, and a Javascript snippet which tests them:

$(function() {      var regex = /&foo(\=[^&]*)?(?=&|$)|^foo(\=[^&]*)?(&|$)/;            var escapeHtml = function (str) {          var map = {            '&': '&amp;',            '<': '&lt;',            '>': '&gt;',            '"': '&quot;',            "'": '&#039;'          };                    return str.replace(/[&<>"']/g, function(m) { return map[m]; });      };              //test cases      var tests = [          'foo'     , 'foo&bar=456'     , 'bar=456&foo'     , 'abc=789&foo&bar=456'         ,'foo='    , 'foo=&bar=456'    , 'bar=456&foo='    , 'abc=789&foo=&bar=456'         ,'foo=123' , 'foo=123&bar=456' , 'bar=456&foo=123' , 'abc=789&foo=123&bar=456'         ,'xfoo'    , 'xfoo&bar=456'    , 'bar=456&xfoo'    , 'abc=789&xfoo&bar=456'         ,'xfoo='   , 'xfoo=&bar=456'   , 'bar=456&xfoo='   , 'abc=789&xfoo=&bar=456'         ,'xfoo=123', 'xfoo=123&bar=456', 'bar=456&xfoo=123', 'abc=789&xfoo=123&bar=456'         ,'foox'    , 'foox&bar=456'    , 'bar=456&foox'    , 'abc=789&foox&bar=456'         ,'foox='   , 'foox=&bar=456'   , 'bar=456&foox='   , 'abc=789&foox=&bar=456'         ,'foox=123', 'foox=123&bar=456', 'bar=456&foox=123', 'abc=789&foox=123&bar=456'      ];            //expected results      var expected = [          ''        , 'bar=456'         , 'bar=456'         , 'abc=789&bar=456'         ,''        , 'bar=456'         , 'bar=456'         , 'abc=789&bar=456'         ,''        , 'bar=456'         , 'bar=456'         , 'abc=789&bar=456'         ,'xfoo'    , 'xfoo&bar=456'    , 'bar=456&xfoo'    , 'abc=789&xfoo&bar=456'         ,'xfoo='   , 'xfoo=&bar=456'   , 'bar=456&xfoo='   , 'abc=789&xfoo=&bar=456'         ,'xfoo=123', 'xfoo=123&bar=456', 'bar=456&xfoo=123', 'abc=789&xfoo=123&bar=456'         ,'foox'    , 'foox&bar=456'    , 'bar=456&foox'    , 'abc=789&foox&bar=456'         ,'foox='   , 'foox=&bar=456'   , 'bar=456&foox='   , 'abc=789&foox=&bar=456'         ,'foox=123', 'foox=123&bar=456', 'bar=456&foox=123', 'abc=789&foox=123&bar=456'      ];            for(var i = 0; i < tests.length; i++) {          var output = tests[i].replace(regex, '');          var success = (output == expected[i]);                    $('#output').append(              '<tr class="' + (success ? 'passed' : 'failed') + '">'              + '<td>' + (success ? 'PASS' : 'FAIL') + '</td>'              + '<td>' + escapeHtml(tests[i]) + '</td>'              + '<td>' + escapeHtml(output) + '</td>'              + '<td>' + escapeHtml(expected[i]) + '</td>'              + '</tr>'          );      }        });

#output {      border-collapse: collapse;        }  #output tr.passed { background-color: #af8; }  #output tr.failed { background-color: #fc8; }  #output td, #output th {      border: 1px solid black;      padding: 2px;  }

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>  <table id="output">      <tr>          <th>Succ?</th>          <th>Input</th>          <th>Output</th>          <th>Expected</th>      </tr>  </table>

Answer 1

If you want to do this in just one regular expression, you could do this:

/&foo(=[^&]*)?|^foo(=[^&]*)?&?/ 

This is because you need to match either an ampersand before the foo=..., or one after, or neither, but not both.

To be honest, I think it's better the way you did it: removing the trailing ampersand in a separate step.