This should do it: "C:\\backup\\".replace(/\\\\/g, '\\')

In the regular expression, a single \ must be escaped to \\, and in the replacement \ also.

[edit 2021] Maybe it's better to use template literals.

console.log(`original solution ${"C:\\backup\\".replace(/\\\\/g, '\\')}`)

// a template literal will automagically replace \\ with \
console.log(`template string without further ado ${`C:\\backup\\`}`);

// but if they are escaped themselves
console.log(`Double escaped ${`C:\\\\backup\\\\`.replace(/\\{2,}/g, '\\')}`);

// multiple escaped
console.log(`multiple escaped ${`C:\\\\\\\\backup\\\\`
  .replace(/\\{2,}/g, '\\')}`);

// don't want to replace the last \\
console.log(`not the last ${`C:\\\\\\backup\\\\`
  .replace(/\\{2,}([^\\{2,}$])/g, (a ,b) => a.slice(0,1) + b)}` );

// don't want to replace the first \\
console.log(`not the first ${`C:\\\\backup\\`.replace(/\\[^\\]$/g, '\\')}`);

// a generic tagged template to replace all multiple \\ OR //
const toSingleSlashes = (strs, ...args) => 
  strs.reduce( (a, v, i ) => 
    a.concat(args[i-1] || ``).concat(v, ``), `` )
      .replace( /(\\|\/){2,}/g, (a, b) => b );

console.log(`generic tagged template 1 ${
  toSingleSlashes`C:\\backup\\`}`);

console.log(`generic tagged template 2 ${
  toSingleSlashes`C:\\\\\\\\backup\\\\\\\\\\`}`);
  
console.log(`generic tagged template 3 ${
  toSingleSlashes`C:\\\\\\\\backup\\`}`);
  
console.log(`generic tagged template 4 ${
  toSingleSlashes`C:////////backup////`}`);

console.log(`reply to comment @chitgoks => 
"test\\\\hehehe" is by default "test\\hehehe"
so, no replacement necessary here ...`);

.as-console-wrapper {
    max-height: 100% !important;
}

Try:

   var parts = replaceableString.split('\\');
   var output = parts.join('\\\\');

Personally, as I am not so expert in reg exps, I tend to avoid them when dealing with non-alphanumeric characters, both due to readability and to avoid weird mistake.

When using regex,

• a literal reverse solidus (a.k.a. backslash,( a.k.a. \)) must be escaped with a \ .

• So when you input \\, that's literally \.

• If you want to input 2 \ each one must be escaped. That means \\\\ is literally a string of 2 \.

The following demo uses ES6 Template Literals:

• It's a new syntax for String that uses the backtick ` (top left corner key) instead of quotes " or '.

• It can interpolate variables by prefixing with ${ and suffixing with }.

  var x = 2; `this template literal will display x as ${x}.`
  // this template literal will display x as 2.
  

• The \ is U+5c in unicode. Using escaped unicode in Template Literal, it would be prefixed with \u{ and suffixed with }

  `A template literal with an escaped unicode backslash is \u{5c}`
  // A template literal with an escaped unicode backslash is \.
  

Demo

function XtoY(str, x, y) {
  var rgx = `/${x}/g`;
  var result = str.replace(rgx, `${y}`);
  return result;
}

const uni = `\u{5c}`;
const string = `\\a\\a\\b`;
const from = `${uni}${uni}`
const to = `${uni}`;

let z = XtoY(string, from, to);

console.log(z);

JavaScript strings need to be escaped so if you are getting a string literal with two back slashes, JavaScript interprets it as just one. In your string you are using to compare, you have \t, which is a tab character, when what you probably want is \\t. My guess is that wherever you are getting the current SharePoint user from, it is being properly escaped, but your compare list isn't.

Edit:

Or maybe the other way around. If you're using .NET 4+ JavaScriptStringEncode might be helpful. If you're still having problems it might help to show us how you are doing the comparison.

You should escape the slash twice:

"tel:\\99999999999".replace('\\\\', '\\');

Actually, the first method is correct and works. Two backslash is indeed used to make a single backslash.

console.log("hello & 100%".replace(/([&%$#_{}])/g, "\\$1")); // "hello \& 100\%"

If console.log is giving you some trouble, which it shouldn't, try alert. Also you need to use the return value which you aren't doing.

How to write it?

You said it yourself:

to display one backslash, I need to write two.

So, if you have two in the string to start with, then you need to replace two (type four) with one (type two).

var filter = "This string has a double slash in it: \\\\";
console.log(`The original string: ${filter}`);
filter = filter.replace("\\\\", "\\");
console.log(`The filtered string: ${filter}`);

Andrew Cooper's answer is correct in terms of why that third statement is going wrong. But you're also overwriting newbigbend each time, so you won't see the result of the first two replacements at all.

If you're trying to replace all slashes, backslashes, and asterisks with nothing, do this:

newbigbend = bb_val.replace(/[/\\*]/g, "");

Note you don't need the i flag, none of those characters is case sensitive anyway. (And note that within the [], you don't need to escape / or *, because they don't have special meaning there.) Live example.

But if you want it as three individual statements for whatever reason, then use newbigbend in the second two (and add the backslash Andrew flagged up):

newbigbend = bb_val.replace(/\//gi,"");
newbigbend = newbigbend.replace(/\\/gi,"");
newbigbend = newbigbend.replace(/\*/gi,"");

Got stumped by this for ages and all the answers kept insisting that the source string needs to already have escaped backslashes in it ... which isn't always the case.

Do this ..

var replaced = str.replace(String.fromCharCode(92),String.fromCharCode(92,92));