Insert space before capital letters

You can just add a space before every uppercase character and trim off the leading and trailing spaces

s = s.replace(/([A-Z])/g, ' $1').trim()

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This will find each occurrence of a lower case character followed by an upper case character, and insert a space between them:

s = s.replace(/([a-z])([A-Z])/g, '$1 $2');

For special cases when 2 consecutive capital letters occur (Eg: ThisIsATest) add additional code below:

 s = s.replace(/([A-Z])([A-Z])/g, '$1 $2');

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Might I suggest a slight edit to the currently accepted answer:

function insertSpaces(string) {
    string = string.replace(/([a-z])([A-Z])/g, '$1 $2');
    string = string.replace(/([A-Z])([A-Z][a-z])/g, '$1 $2')
    return string;
}

This means that:

ACROText -> ACRO Text
UserNameTest -> User Name Test

Which might be slightly more useful if you are dealing with db column names (And are using acronyms for some things)

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Here is what i ended up using to convert a string to a title case, based on a few of the answers here:

str = str
  .replace(/(_|-)/g, ' ')
  .trim()
  .replace(/\w\S*/g, function(str) {
    return str.charAt(0).toUpperCase() + str.substr(1)
  })   
  .replace(/([a-z])([A-Z])/g, '$1 $2')
  .replace(/([A-Z])([A-Z][a-z])/g, '$1 $2')   

Here is a JSFiddle where you can test your string to see if this meets your needs: https://jsfiddle.net/thomastasa/5236dv8t/85/

Examples:

"yourStringHere" -> "Your String Here"

"AnotherStringHere" -> "Another String Here"

"someones_string" -> "Someones String"

"Another-String-Here" -> "Another String Here"

"myAWESOMEString" -> "My AWESOME String"

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This should insert a space between each capital letter that was not preceded by a capital letter.

var myString = "MySites"
var newString = "";
var wasUpper = false;
for (var i = 0; i < myString.length; i++)
{
    if (!wasUpper && myString[i] == myString.toUpperCase()[i])
    {
        newString = newString + " ";
        wasUpper = true;
    }
    else
    {
        wasUpper = false;
    }
    newString = newString + myString[i];
}

newString will have the value you want. Also, if you want to shorten your code using regex, you can use the following code from Javascript camelCase to Regular Form

"thisStringIsGood"
    // insert a space before all caps
    .replace(/([A-Z])/g, ' $1')
    // uppercase the first character
    .replace(/^./, function(str){ return str.toUpperCase(); })

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regex to find lower case - upper case boundary then insert a space

<div id='x'>ThisIsMySites</div>
$('#x').text( $('#x').text().replace(/([a-z])([A-Z])/g, "$1 $2") );

http://jsfiddle.net/uXy64/

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This is another possibility, way much compact

export function replaceCamelSpaces(colorName){
   return colorName.replace(/\B([A-Z])\B/g, ' $1');
}

Explained :

\B uppercase word boundary to find, for more read this

([A-Z]) finding zero, one or more occurence of Upper letters

\B as before but at the end of the search

/g match all occurences of previous matches

replace with

' $1'

Replace with some letter(lowacase) and a space at beginning

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You can use String#split() and a look-ahead for the capitalized alphabet ([A-Z]) and then Array#join() the array with a space:

let stringCamelCase = "MySites"; let string = stringCamelCase.split(/(?=[A-Z])/).join(" "); console.log(string)

Or, as a String Object function:

String.prototype.cC2SC = function() { return this.split(/(?=[A-Z])/).join(" "); } console.log("MyCamelCase".cC2SC());

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In a single regex replace (without trims or joins) that allows any character not just letters [a-z] or [A-Z].

const str = "MySites";
str.replace(/(?<!^)([A-Z])/, " $1"); // -> "My Sites"

You can check more about the look behind (?<!^) here.

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