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select particular input field name of particular form

i have some html code like this
<form name="first"><input name="firstText" type="text" value="General" />
<input name="secondText" type="text" value="General" />
<input name="ThirdText" type="text" value="General" />
<input name="FourthText" type="text" value="General" />
<input name="FifthText" type="text" value="General" />
</form>
<form name="second"><input name="firstText" type="text" value="General" />
<input name="secondText" type="text" value="General" />
<input name="ThirdText" type="text" value="General" />
<input name="FourthText" type="text" value="General" />
<input name="FifthText" type="text" value="General" />
</form>
i want to select "secondText" of form "second" using jquery or javascript and i want to change value of it using jquery.

Using jQuery:
var element = $("form[name='second'] input[name='secondText']");
Using vanilla JS:
var element = document.querySelector("form[name='second'] input[name='secondText']");
Changing the value: element.val(value) or element.value = value, depending of what you are using.

To the point with pure JS:
document.querySelector('form[name=particular-form] input[name=particular-input]')
Update:
This selector will return the input named "particular-input" inside form named "particular-form" if exists, otherwise returns null.
The selector filter "form[name=particular-form]" will look for all forms with name equals "particular-form":
<form name="particular-form">
The selector filter "input[name=particular-input]" will look for all input elements with name equals "particular-input":
<input name="particular-input">
Combining both filters with a white space, I mean:
"form[name=particular-name] input[name=particular-input]"
We are asking for querySelector(): Hey, find all inputs with name equals "particular-input" nested in all forms with name equals "particular-form".
Consider:
<form name="particular-form">
<input name="generic-input">
<input name="particular-input">
</form>
<form name="another-form">
<input name="particular-input">
</form>
<script>
document.querySelector('form[name=particular-form] input[name=particular-input]').style.background = "#f00"
</script>
This code will change the background color only of the second input, no matter the third input have same name. It is because we are selecting only inputs named "particular-input" nested in form named "particular form"
I hope it's more clear now.
;)
By the way, unfortunately I didn't found good/simple documentation about querySelector filters, if you know any reference, please post here.

// Define the target element
elem = jQuery( 'form[name="second"] input[name="secondText"]' );
// Set the new value
elem.val( 'test' );

Try
$("form[name='second'] input[name='secondText']").val("ENTER-YOUR-VALUE");

You can do it like this:
jQuery
$("form[name='second'] input[name='secondText']").val("yourNewValue");
Demo: http://jsfiddle.net/YLgcC/
Or:
Native Javascript
Old browsers:
var myInput = [];
myInput = document.getElementsByTagName("input");
for (var i = 0; i < myInput.length; i++) {
if (myInput[i].parentNode.name === "second" &&
myInput[i].name === "secondText") {
myInput[i].value = "yourNewValue";
}
}
Demo: http://jsfiddle.net/YLgcC/1/
New browsers:
document.querySelector("form[name='second'] input[name='secondText']").value = "yourNewValue";
Demo: http://jsfiddle.net/YLgcC/2/

You can try this line too:
$('input[name="elements[174ec04d-a9e1-406a-8b17-36fadf79afdf][0][value]"').mask("999.999.999-99",{placeholder:" "});

Add button in both forms. On Button click find nearest form using closest() function of jquery. then using find()(jquery function) get all input values. closest() goes in upward direction in dom tree for search and find() goes in downward direction in dom tree for search. Read here
Another way is to use sibling() (jquery function). On button click get sibling input field values.

How do I replace part of the value of attributes for a set of elements in jQuery?

I am trying to replace a series of 'for' attributes of labels based on their current contents.
The application is using AJAX to add an item to an invoice without refreshing the page. Upon receiving notification of a successful item add, my script should replace all the labels in the form whose 'for' attribute ends with '-new' with the same attribute minus the '-new' and adding ('-' + itemValue), where itemValue is the item Id of the invoice item that was added.
I know how to select all the labels I want to change at once:
jQuery('label[for$=new]')
I know how to get their 'for' attribute:
jQuery('label[for$=new]').attr('for')
I tried the JavaScript replace method:
jQuery('label[for$=new]').attr('for').replace(/-new/,itemValue)
But that appears to select each label's 'for' attribute, replace the text, and pass the replaced text back (to nothing), since I don't know how to identify the labels that have the 'for' attribute I want to replace.
Here's some sample HTML:
<form id="InvoiceItemsForm-1" action="<?=$_SERVER['PHP_SELF'];?>" method="post" name="InvoiceItemsForm-1" onsubmit="return false">
<div id="InvoiceItem-new-1" class="InvoiceItem">
<label for="InvoiceItemNumber-new">New Invoice Item Number: </label>
<input id="InvoiceItemNumber-new" class="InvoiceItemNumber" type="text" value="" name="InvoiceItemNumber-new">
<label for="InvoiceItemDescription-new">Item Description: </label>
<input id="InvoiceItemDescription-new" class="InvoiceItemDescription" type="text" value="" name="InvoiceItemDescription-new">
<label for="InvoiceItemAmount-new">Item Amount: </label>
<input id="InvoiceItemAmount-new" class="InvoiceItemAmount" type="text" value="" name="InvoiceItemAmount-new">
<input id="addInvoiceItem-1" width="25" type="image" height="25" src="/payapp/images/greenplus.th.png" alt="Add New Invoice Item" onclick="addInvoiceItemButtonPushed(this)" value="invoiceItem">
</div>
<button id="CloseInvoice-1" onclick="closeInvoice(this)" type="button">Close Invoice</button>
</form>
Once I get this to work, I'm going to replace all the ids for all the inputs. Same problem. I imagine the solution looks something like this:
jQuery('input[id$=new]').attr('id').replace(/-new/,itemValue)
I just cannot figure out the syntax for this at all.

No need to use .each() ... the .attr() method accepts a function as the second parameter that returns the new value to be used as replacement
jQuery('label[for$=new]').attr('for', function(index, currentValue){
return currentValue.replace(/-new/,'-' + itemValue);
});

If I may, why not just put the input tag inside the label tag? That way, you won't need a for attribute inside the label tag.
Next, a better way to accomplish what you're trying to do would be to use the invoice ID number as the ID for the surrounding div, and add a 'new` class for "new" invoice entries.
So your form would look something like this:
<form id="InvoiceItemsForm-1" action="<?=$_SERVER['PHP_SELF']" method="post" name="InvoiceItemsForm-1" onsubmit="return false">
<div class="InvoiceItem new">
<label>New Invoice Item Number: <input class="InvoiceItemNumber" type="text" value="" name="InvoiceItemNumber"></label>
<label>Item Description: <input class="InvoiceItemDescription" type="text" value="" name="InvoiceItemDescription-new"></label>
<label for="InvoiceItemAmount-new">Item Amount: <input class="InvoiceItemAmount" type="text" value="" name="InvoiceItemAmount-new"></label>
<input id="addInvoiceItem-1" width="25" type="image" height="25" src="/payapp/images/greenplus.th.png" alt="Add New Invoice Item" onclick="addInvoiceItemButtonPushed(this)" value="invoiceItem">
</div>
<button id="CloseInvoice-1" onclick="closeInvoice(this)" type="button">Close Invoice</button>
</form>
You'll still have all the targetability you need to get the new invoice item field data, but now, you only have two things to do to convert from a "new" invoice row to an "existing" invoice item row: add an id attribute to the div and remove the new class, both of which jQuery will let you do quite easily.

Not sure I get the question, but something like:
var oldFor = $('label[for$=new]').attr('for');
var newFor = oldfor.replace(/-new/,itemValue);
$('label[for$=new]').attr('for', newFor);
.attr( attributeName, value )
attributeName = The name of the attribute to set.
value = A value to set for the attribute.
When selecting multiple elements, you will need to iterate:
$('label[for$=new]').each(function(index) {
$(this).attr('for', $(this).attr('for').replace(/-new/, '-' + itemValue));
});

jQuery val() method not working

I wanted to read the value entered in the text box in one of my HTML form, for this I tried jQuery val() method, but it is not working, any idea why?
HTML:
<form method="POST" id="payment-form">
<p>
<label class="card-number" for="txt_cardno"><span>Card Number:</span></label>
<input type="text" size="20" autocomplete="off" class="card-number" id="txt_cardno" name="cardno" />
</p>
<p class="submit submit-button"><a class="btn" href="#">Charge Card</a><br><a class="btn" href="#" onClick="return false">Go Back</a></p>
<div class="clear"></div>
</form>
Javascript:
$(document).ready(function() {
$(".submit").live("click", function(e) {
e.preventDefault();
var card_num = $('.card-number').val();
alert(card_num);
});
});
the jsfiddle is http://jsfiddle.net/74neK/1/

Use the id to access it (faster):
var card_num = $('#txt_cardno').val();

You've given both the <label> and the <input> the "card-number" class.
Specify the input in the selector. Otherwise .val() gives you the value of the first element found (the label).
var card_num = $('input.card-number').val();
http://jsfiddle.net/74neK/3/
If you're really concerned with micro-optimization, you should use native methods:
var card_num = (document.getElementById('txt_cardno')||{}).value;
http://jsfiddle.net/74neK/5/

Your <label>'s class is the same as your <input>'s, so jQuery is trying and failing to retrieve the value of your <label>. Instead, refer to your <input> by name or id:
$('#txt_cardno').val()
I would recommend ID regardless, because jQuery optimizes it to document.getElementById, which is much faster.

Try to use the ID to access the element:
var card_num = $('#txt_cardno').val();
http://jsfiddle.net/74neK/4/ <- Example

Select the id of your input element, as opposed to the class: http://jsfiddle.net/btgxu/

Adding "i" to input field, but should be hidden

I want to add "i" to a input field when the red div is clicked, but the "i" that is added to the input field should not be viewable. If the green button is clicked the hidden "i" should be removed.
Here is my HTML live: http://jsfiddle.net/mtYtW/60/
My HTML:
<div class="input string optional">
<label for="company_navn" class="string optional">Name</label>
<input type="text" size="50" name="company[navn]" maxlength="255" id="webhost_navn" class="string optional">
</div>
<div style="width:30px;height:30px;margin-top:10px;display:block;background:green;">
</div>
<div style="width:30px;height:30px;margin-top:10px;display:block;background:red;">
</div>
How to create this functionality?

If you would like to associate data with a specific element, I suggest the .data() method of jQuery. Take a look at the jQuery docs. It's a much cleaner way of accomplishing your goal.
Here's a working Fiddle to get you started.
EDIT
Per the new requirement spelled out in the comments to your question, you can attach to the form submit event like this:
$('#yourForm').submit(function() {
if($('#webhost_navn').data('myData') == 'i')
{
var val = $('#webhost_navn').val();
$('#webhost_navn').val('i' + val);
}
});
NOTE: This code relys on the orginal code in my Fiddle.

It sounds like you want to associate some data with the input field, but not alter the input field's value. For that, you can use the data method:
$(document).ready(function() {
$('#redDiv').click(function() {
$('#webhost_navn').data('myData', 'i');
});
$('#greenDiv').click(function() {
$('#webhost_navn').data('myData', null);
});
});
You'll need to add id's to the red and green divs for the above example to work as is, respectively, redDiv and greenDiv. To retrieve the data you associate with the input, do this:
var myData = $('#webhost_navn').data('myData'); // Will equal 'i' or null
API Ref: http://api.jquery.com/data
EDIT: To append the "i" value to the input's value:
var myData = $('#webhost_navn').data('myData'),
val = $('#webhost_navn').val();
if (myData) {
$('#webhost_navn').val(myData + val);
}
Working example: http://jsfiddle.net/FishBasketGordo/e3yKu/

My update to your code here: http://jsfiddle.net/mtYtW/61/
Basically I gave your red/green button's id's and created a click event to add/remove the content. I also created a css definition for the color of the input box to be white so you don't see the text.
<div class="input string optional"><label for="company_navn" class="string optional"> Name</label><input type="text" size="50" name="company[navn]" maxlength="255" id="webhost_navn" class="string optional"></div>
<div id='green' style="width:30px;height:30px;margin-top:10px;display:block;background:green;"></div>
<div id='red' style="width:30px;height:30px;margin-top:10px;display:block;background:red;"></div>
css:
label {display:block;}
#webhost_navn{color:white};
js:
$("#red").live("click",function()
{
$("#webhost_navn").val("i");
});
$("#green").live("click",function()
{
$("#webhost_navn").val("");
});
Note if the goal is to post an "i" and have nothing else as a value (ie no user input) use <input type='hidden' id=webhost_navn > and use the same jquery code as above without the need for the css.

retrieving text field value using javascript

I want to retrieve textfield value using javascript. suppose i have a code like:
<input type='text' name='txt'>
And I want to retrieve it using javascript. I call a function when a button is clicked:
<input type='button' onclick='retrieve(txt)'>
What coding will the retrieve function consist of?

You can do this:
Markup:
<input type="text" name="txt" id="txt"/>
<input type="button" onclick="retrieve('txt');"/>
JavaScript:
function retrieve(id) {
var txtbox = document.getElementById(id);
var value = txtbox.value;
}

Let's say you have an input on your page with an id of input1, like this:
<input type="text" id="input1" />
You first need to get the element, and if you know the Id, you can use document.getElementById('input1'). Then, just call .value to get the value of the input box:
var value = document.getElementById('input1').value;
Update
Based on your markup, I would suggest specifying an id for your text box. Incase you don't have control over the markup, you can use document.getElementsByName, like so:
var value = document.getElementsByName('txt')[0].value;

One of the way is already explained by Andrew Hare.
You can also do it by entering the value in the textbox and getting a prompt box with entered message when a user click the button.
Let's say, you have a textbox and a input button
<input type="text" name="myText" size="20" />
<input type="button" value="Alert Text" onclick="retrieve()" />
The function for retrieve()
function retrieve()
{
var text = document.simpleForm.myText.value;
alert(text);
}