What characters are allowed in the HTML Name attribute inside input tag?

Note, that not all characters are submitted for name attributes of form fields (even when using POST)!

White-space characters are trimmed and inner white-space characters as well the character . are replaced by _. (Tested in Chrome 23, Firefox 13 and Internet Explorer 9, all Win7.)

Any character you can include in an [X]HTML file is fine to put in an <input name>. As Allain's comment says, <input name> is defined as containing CDATA, so the only things you can't put in there are the control codes and invalid codepoints that the underlying standard (SGML or XML) disallows.

Allain quoted W3 from the HTML4 spec:

Note. The "get" method restricts form data set values to ASCII characters. Only the "post" method (with enctype="multipart/form-data") is specified to cover the entire ISO10646 character set.

However this isn't really true in practice.

The theory is that application/x-www-form-urlencoded data doesn't have a mechanism to specify an encoding for the form's names or values, so using non-ASCII characters in either is “not specified” as working and you should use POSTed multipart/form-data instead.

Unfortunately, in the real world, no browser specifies an encoding for fields even when it theoretically could, in the subpart headers of a multipart/form-data POST request body. (I believe Mozilla tried to implement it once, but backed out as it broke servers.)

And no browser implements the astonishingly complex and ugly RFC2231 standard that would be necessary to insert encoded non-ASCII field names into the multipart's subpart headers. In any case, the HTML spec that defines multipart/form-data doesn't directly say that RFC2231 should be used, and, again, it would break servers if you tried.

So the reality of the situation is there is no way to know what encoding is being used for the names and values in a form submission, no matter what type of form it is. What browsers will do with field names and values that contain non-ASCII characters is the same for GET and both types of POST form: it encodes them using the encoding the page containing the form used. Non-ASCII GET form names are no more broken than everything else.

DLH:

So name has a different data type for than it does for other elements?

Actually the only element whose name attribute is not CDATA is <meta>. See the HTML4 spec's attribute list for all the different uses of name; it's an overloaded attribute name, having many different meanings on the different elements. This is generally considered a bad thing.

However, typically these days you would avoid name except on form fields (where it's a control name) and param (where it's a plugin-specific parameter identifier). That's only two meanings to grapple with. The old-school use of name for identifying elements like <form> or <a> on the page should be avoided (use id instead).

The only real restriction on what characters can appear in form control names is when a form is submitted with GET

"The "get" method restricts form data set values to ASCII characters." reference

There's a good thread on it here.

While Allain's comment did answer OP's direct question and bobince provided some brilliant in-depth information, I believe many people come here seeking answer to more specific question: "Can I use a dot character in form's input name attribute?"

As this thread came up as first result when I searched for this knowledge I guessed I may as well share what I found.

Firstly, Matthias' claimed that:

character . are replaced by _

This is untrue. I don't know if browser's actually did this kind of operation back in 2013 - though, I doubt that. Browsers send dot characters as they are(talking about POST data)! You can check it in developer tools of any decent browser.

Please, notice that tiny little comment by abluejelly, that probably is missed by many:

I'd like to note that this is a server-specific thing, not a browser thing. Tested on Win7 FF3/3.5/31, IE5/7/8/9/10/Edge, Chrome39, and Safari Windows 5, and all of them sent " test this.stuff" (four leading spaces) as the name in POST to the ASP.NET dev server bundled with VS2012.

I checked it with Apache HTTP server(v2.4.25) and indeed input name like "foo.bar" is changed to "foo_bar". But in a name like "foo[foo.bar]" that dot is not replaced by _!

My conclusion: You can use dots but I wouldn't use it as this may lead to some unexpected behaviours depending on HTTP server used.