Stackoverflow 2012-03-15

HTML

<form id="form1" action="http://www.domain.com/processing.php?page=first&amp;section=77&amp;userid=  ">
</form>

<form id="form2" action="http://www.domain.com/processing.php?page=first&amp;userid=replaceme&amp;section=77">
</form>

<form id="form3" action="http://www.domain.com/processing.php?userid=replaceme&amp;page=first&amp;section=77">
</form>

<form id="form4" action="http://www.domain.com/processing.php?page=first&amp;superuserid=replaceme&amp;section=77">
</form>

<form id="form5" action="http://www.domain.com/processing.php?page=first&amp;section=77&amp;userid=replaceme#tag">
</form>

<div id="output1"></div>
<div id="output2"></div>
<div id="output3"></div>
<div id="output4"></div>
<div id="output5"></div>

JavaScript

var newValue = 'test';

function replaceUserId($form) {
    $form.prop('action', function(i, action){ return action.replace(/([\?&])(userid=)[^&#]*/, '$1$2' + newValue); });
}

replaceUserId($('#form1'));
replaceUserId($('#form2'));
replaceUserId($('#form3'));
replaceUserId($('#form4'));
replaceUserId($('#form5'));

$('#output1').text($('#form1').prop('action'));
$('#output2').text($('#form2').prop('action'));
$('#output3').text($('#form3').prop('action'));
$('#output4').text($('#form4').prop('action'));
$('#output5').text($('#form5').prop('action'));