input.replace(/[^\w\s]/gi, '')

Shamelessly stolen from the other answer. ^ in the character class means "not." So this is "not" \w (equivalent to \W) and not \s, which is space characters (spaces, tabs, etc.) You can just use the literal if you need.

Removing non-alphanumeric chars

The following is the/a correct regex to strip non-alphanumeric chars from an input string:

input.replace(/\W/g, '')

Note that \W is the equivalent of [^0-9a-zA-Z_] - it includes the underscore character. To also remove underscores use e.g.:

input.replace(/[^0-9a-z]/gi, '')

The input is malformed

Since the test string contains various escaped chars, which are not alphanumeric, it will remove them.

A backslash in the string needs escaping if it's to be taken literally:

"\\test\\red\\bob\\fred\\new".replace(/\W/g, '')
"testredbobfrednew" // output

Handling malformed strings

If you're not able to escape the input string correctly (why not?), or it's coming from some kind of untrusted/misconfigured source - you can do something like this:

JSON.stringify("\\test\red\bob\fred\new").replace(/\W/g, '')
"testredbobfrednew" // output

Note that the json representation of a string includes the quotes:

JSON.stringify("\\test\red\bob\fred\new")
""\\test\red\bob\fred\new""

But they are also removed by the replacement regex.

Remove the space from the first set and will do the job:

name.replace(/[^0-9a-zA-Z]/g, '');

You may read this code as "remove all characters that are not digits ([0-9]) and alpha characters ([a-zA-Z])".

Alternatively, you can use the i flag to make your regular expression ignore case. Then the code can be simplified:

name.replace(/[^0-9a-z]/gi, ''); 

Be aware, that \W leaves the underscore. A short equivalent for [^a-zA-Z0-9] would be [\W_]

text.replace(/[\W_]+/g," ");

\W is the negation of shorthand \w for [A-Za-z0-9_] word characters (including the underscore)

Example at regex101.com

This is actually fairly straightforward.

Assuming str is the string you're cleaning up:

str = str.replace(/[^a-z0-9+]+/gi, '+');

The ^ means "anything not in this list of characters". The + after the [...] group means "one or more". /gi means "replace all of these that you find, without regard to case".

So any stretch of characters that are not letters, numbers, or '+' will be converted into a single '+'.

To remove parenthesized substrings (as requested in the comments), do this replacement first:

str = str.replace(/\(.+?\)/g, '');

function replacer() {

  var str = document.getElementById('before').value.
    replace(/\(.+?\)/g, '').
    replace(/[^a-z0-9+]+/gi, '+');

  document.getElementById('after').value = str;
}

document.getElementById('replacem').onclick = replacer;

<p>Before:
  <input id="before" value="Durarara!!x2 Ten" />
</p>

<p>
  <input type="button" value="replace" id="replacem" />
</p>

<p>After:
  <input id="after" value="" readonly />
</p>

At least in other languages, invoking the regular expressions engine is expensive. I'm not sure if that's true of JavaScript, but here's how you'd do it "C-style". I'm sure benchmarking its performance yourself will be a valuable learning experience.

var x = "Well Done!";
var y = "";
var c;
for (var i = 0; i < x.length; i++)
{
    c = x.charCodeAt(i);
    if (c >= 48 && c <= 57 || c >= 97 && c <= 122)
    {
        y += x[i];
    }
    else if (c >= 65 && c <=  90)
    {
        y += String.fromCharCode(c+32);
    }
    else if (c == 32 || c >= 9 && c <= 13)
    {
        y += '-';
    }
}
$('#output').html(y);

See http://www.asciitable.com/ for ASCII codes. Here's a jsFiddle. Note that I've also implemented your toLowerCase() simply by adding 32 to the uppercase letters.

Disclaimer

Personally of course, I prefer readable code, and therefore prefer regular expressions, or using some kind of a strtr function if one exists in JavaScript. This answer is purely to educate.

You can use a negated character class:

var temp = originalname.replace(/[^\w.]+/g, "_");

[^\w.]+ will match 1 or more of any character that is not a word character and not a DOT.