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Replace HTML string with javascript using .replace

I have this html code:
<p>Hello, this is a test replacing, <span class="myclass">over test</span> and <span class="myclass">over test</span>.</p>
My javascript works to replace the word "Hello" with "FuuBar".
document.body.innerHTML = document.body.innerHTML.replace(/Hello/g, "FuuBar");
But I can not replace <span class="myclass">over test</span> by <span class="thanks"><b>Thanks God</b></span>
I am starting in javascript. I need to resolve this in pure js. Could help in my code? And sorry for the English.
JsFiddle for help.

Use the DOM for this. Check here for more info: https://developer.mozilla.org/en-US/docs/DOM
var spans = document.querySelectorAll('.myclass');
for (var i=0; i<spans.length; i++) {
spans[i].classList.remove('myclass');
spans[i].classList.add('thanks');
spans[i].innerHTML = '<b>Thanks god</b>';
}

Have you checked quotations? You should escape them in the classname in JS or use single quotes. Please provide the code if that's not the case.
Other than that, replacing the whole body doesn't seem the best idea for a task like this.

Concatenate Two strings in HTML in client side(No Server)

Can I concatenate two strings in HTML?
I want to achieve the following functionality-
go to the 1st DIV tag.
It could have been done using document.write() in javascript but I want to know if there is any concatenation functionality in HTML itself.

No, there isn't.
HTML is markup, it is not turing complete.

One (primitive) way to achieve this with JavaScript would be
<a href="#"
onclick="window.location.hash='#'+document.getElementsByTagName('div')[0].id; return false;">
go to the 1st DIV tag.
</a>
But since those links are useless when JS is not available, they should probably only be generated by JS in the first place.

No, there isn't. HTML is markup.
You should use dynamic HTML and JavaScript to achieve this.

you can do this by using this.href in java script
<a href="#" onload="this.href=this.href+document.getElementsByTagName('div')[0].id;" >
ex
<a href="targetWithInDoc.html" onload="this.href=this.href+'#block1';" >block 1</a>

This can't be done in the way you're attempting, but if JavaScript is running on the client anyway then you can still achieve the functionality you're looking for. You just need to separate the tag from the script:
Go to the first DIV tag
<script type="text/javascript">
document.getElementById('someID').href = '#' + document.getElementsByTagName('div')[0].id;
</script>

I know it wont help u now but I'm posting this for others who will come to this question by searching
we can achieve it this way :
<a href='<%#String.Concat("string1", "string2")%>'></a>

Removing everything between two strings regex

I have the following:
<span class="hello" id="something somemore"> blah blah blah </span>
My regex is not my strongest ability, would appreciate is someone could help me in javascript regex.
I would like to remove everything/anything and including from <span to > and then finally just remove the </span>.
I am using .replace(myregexhere, ""); to replace other items but I am struggling to do the above.
Also if there are easier or better alternatives let me know.

If you really want to use regex replace, try this:
var newstring = oldstring.replace(/<span[^>]*>([^<]*)<\/span>/, "$1");
Note that this doesn't work if any html tags are in between <span> and </span>. If you have html tags there then please use something like this instead.

You want something like:
~<span.+?>~
and
~</span>~

Why not use replaceWith() instead :
var something = $('#something').text();
$('#something').replaceWith(something);

use jQuery:
$('span').remove();
or for each span with class "hello":
$('span .hello').remove();
or for each element with id "something somemore",
$("#something somemore").remove();

Substring html tag using Javascript

I want to sub string and remove the , which appears within the span tag and display the name alone. Below are the two cases which needs to work.
Case1: <span class="datatableheader">No results found, </span>
Case2: <span class="datatableheader">Jude Gomes, </span>
A single function should help in removing the , in both cases and display the result as
<span class="datatableheader">No results found </span>
<span class="datatableheader">Jude Gomes </span>
Appreciate for any help.
Thanks

$(".datatableheader").html ($(".datatableheader").html().replace(",",""));

It's not widely recognized that .html accepts a callback function:
$('.datatableheader').html(function(i,old) {
return old.replace(/, ?/g, '');
});​
http://jsfiddle.net/3fBY4/1/

you can try this also
var parts = id.split(':'); //because u have case1: or case2:
// it will split into string in array//
$('#parts[1]').replace(",",""));
//try to print that it will work
nice question.

how to remove tags with JavaScript regular expressions

I have a JavaScript string containing HTML like this:
<div>
<div class="a">
content1
</div>
content 2
<div class="a">
<b>content 3</b>
</div>
</div>
and I want to remove the div's of class="a" but leave their content.
In Python I would use something like:
re.compile('<div class="a">(.*?)</div>', re.DOTALL).sub(r'\1', html)
What is the equivalent using Javascript regular expressions?

Why don't you use proper DOM methods? With a little help from jQuery, that's dead simple:
var contents = $('<div><div class="a">content1</div>content 2<div class="a"><b>content 3</b></div></div>');
contents.find('.a').each(function() {
$(this).replaceWith($(this).html());
});

You can achieve it with regular expressions in JavaScript
var html = '<div> <div class="a"> content1 </div> <div class="a"> content1 </div> ... </div>';
var result = html.replace(/<div class="a">(.*?)<\/div>/g, function(a,s){return s;});
alert(result);
RegExp method replace takes two parameters - first one is the actual re and the second one is the replacement. Since there is not one but unknown number of replacements then a function can be used.

If you want to do this in Javascript, I'm presuming that you are running it in a web browser, and that the 'javascript string' that you refer to was extracted from the DOM in some way.
If both of these case are true, then I'd say that it would be a good idea to use a tried and tested javascript library, such as JQuery (There are others out there, but I don't use them, so can't really comment)
JQuery allows you to do on-the-fly DOM manipulations like you describe, with relative ease...
$('div.a').each(function(){$(this).replaceWith($(this).html());});
JQuery is definitely one of those tools that pays dividends - a failry short learning curve and a whole lot of power.