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js Split array add space between words (not first)

I have a string Topic: Computer Science
And want to strip out topic: (but in fact I want this to work with any header on the string line) and return Computer Science.
I thought about splitting the components and then adding the spaces back in:
var subjectLine = thisLine.split(" ");
var subjectString = "";
for (i = 1; i < subjectLine.length; i++) {
subjectString += subjectLine[i] + " ";
}
But then I need to remove the last space from the string.
For each doesn't work as I need to NOT have the first element appended.
I'm not sure how to do this in js so it is reusable for many different lines and topic names that can come from the subjectLine

After splitting the line, remove the first element from the array, then join the rest back together.
var thisLine = "Topic: Computer Science";
var subjectLine = thisLine.split(" ");
subjectLine.splice(0, 1);
var subjectString = subjectLine.join(" ");
console.log(subjectString);

You might consider using a regular expression, it'll probably be a lot easier than working with arrays: match the non-space characters at the beginning of the string, followed by at least one space, and .replace with the empty string:
const subjectString = thisLine.replace(/^\S+\s+/, '');
const transform = line => line.replace(/^\S+\s+/, '');
console.log(transform('Topic: Computer Science'));

You need to know where the heading stops and the real data starts. Then delete all characters up to that point.
So, for instance, if you know that the heading ends with a colon, then do:
var line = "this is the topic: Computer Science";
var topic = line.replace(/^.*:\s*/, "");
console.log(topic);

How can I cut the word after certain symbols?

I have such a structure "\"item:Test:3:Facebook\"" and I need somehow fetch the word Facebook.
The words can be dynamic. So I need to get word which is after third : and before \
I tried var arr = str.split(":").map(item => item.trim()) but it doesn't do what I need. How can I cut a word that will be after third : ?

A litte extra code to remove the last " aswell.
var str = "\":Test:3:Facebook\"";
var arr = str.split(":").map(item => item.trim());
var thirdItem = arr[3].replace(/[^a-zA-Z]/g, "");
console.log(thirdItem);

If the amount of colons (:) doesn't vary you can simply use an index on the resulting array like this:
var foo = str.split(":")[3];

The word after the 3rd : will be the fourth word returned, so it will be at index 3 in the array returned by split() (arrays being zero-indexed, of course). You might also want to get rid of the trailing quote mark.
Demo:
str = "\"item:Test:3:Facebook\"";
var word = str.split(":")[3].replace("\"", "");
console.log(word);

This should do the trick, plus remove all symbols
var foo = str.split(":")[3].replace(/[^a-zA-Z ]/g, "")

Get all words starting with X and ending with Y

I have got a textarea with keyup=validate()
I need a javascript function that gets all words starting with # and ending with a character that is not A-Za-z0-9
For example:
This is a text #user1 this is more text #user2. And this is even more #user3!
The function gives an array:
Array("#user1","#user2","#user3");
I am sure there must be a way to do this written on somewhere on the internet if I just google something but I have no idea what I have to look for.. I am very new with regular expresions.
Thank you very much!

The regular expression you want is:
/#[a-z\d]+/ig
This matches # followed by a sequence of letters and numbers. The i modifier makes it case-insensitive, so you don't have to put A-Z in the character class, and g makes it find all the matches.
var str = "This is a text #user1 this is more text #user2. And this is even more #user3!";
var matches = str.match(/#[a-z\d]+/ig);
console.log(matches);

JS
var str = "This is a text #user1 this is more text #user2. And this is even more #user3!",
var textArr = str.split(" ");
for(var i = 0; i < textArr.length; i++) {
var test = textArr[i];
matches = test.match(/^#.*.[A-Za-z0-9]$/);
console.log(matches);
};
Explanation:
You should also read about the regex(http://www.w3schools.com/jsref/jsref_obj_regexp.asp) and match(http://www.w3schools.com/jsref/jsref_match.asp) to get an idea how it works.
Basically, applying ^# means starting the regex look for #. $ means ending with. and .* any character in between.
To Test: http://www.regular-expressions.info/javascriptexample.html

Thanks for the replies above, they've helped me - Where I've written this method that hopefully answers the question about having a start and end regex check.
In this example it looks for ##_ at the start and _## at the end
e.g. ##_ anyTokenYouNeedToFind _##.
Code:
const tokenSearchHelper = (inputText) => {
let matches = inputText.match(/##_[a-zA-Z0-9_\d]+_##/ig);
return matches;
}
const out = tokenSearchHelper("Hello ##_World_##");
console.log(out);

Split in to Sentences and Wrap With Tags

I'm trying to build a text fixing page for normalising text written in all capital letters, all lower case or an ungrammatical mixture of both.
What I'm currently trying to do is write a regular expression to find all full stops, question marks and line breaks, then split the string in to various strings containing all of the words up to and including each full stop.
Then I'm going to wrap them with <span> tags and use CSS :first-letter and text-transform:capitalize; to capitalise the first letter of each sentence.
The last stage will be writing a dictionary function to find user-specified words for capitalisation.
This question only concerns the part about writing a regex and splitting in to strings.
I've tried too many methods to post here, with varying results, but here's my current attempt:
for(var i=0; i < DoIt.length; i++){
DoIt[i].onclick = function(){
var offendingtext = input.value.toString();
var keeplinebreaks = offendingtext.replace(/\r?\n/g, '<br />');
var smalltext = keeplinebreaks.toLowerCase();
//split at each character I specify
var breakitup = smalltext.split(/[/.?\r\n]/g);
breakitup.forEach(function(i){
var i;
console.log(i);
var packagedtogo = document.createElement('span');
packagedtogo.className = 'sentence';
packagedtogo.innerHTML = breakitup[i];
output.appendChild(packagedtogo);
i++;
});
}
}
It was splitting at the right places before, but it was printing undefined in the output area between the tags. I've been at this for days, please could someone give me a hand.
How can I split a string in to multiple string sentences, and then wrap each string with html tags?

Your regex for the split is fine. Just forgot to escape a few characters:
var str = "SDFDSFDSF?sdf dsf sdfdsf. sdfdsfsdfdsfdsfdsfdsfsdfdsf sdf."
str.split( (/[\.\?\r\n]/g))
//["SDFDSFDSF", "sdf dsf sdfdsf", " sdfdsfsdfdsfdsfdsfdsfsdfdsf sdf", ""]
Use for each iteration capabilities like this:
breakitup.forEach(function(element){
var packagedtogo = document.createElement('span');
packagedtogo.className = 'sentence';
packagedtogo.innerHTML = element;//breakitup is undefiend
output.appendChild(packagedtogo);
//No need to increase index
});

split string based on a symbol

I'm trying to split a string into an array based on the second occurrence of the symbol _
var string = "this_is_my_string";
I want to split the string after the second underscore. The string is not always the same but it always has 2 or more underscores in it. I always need it split on the second underscore.
In the example string above I would need it to be split like this.
var split = [this_is, _my_string];

var string = "this_is_my_string";
var firstUnderscore = string.indexOf('_');
var secondUnderscore = string.indexOf('_', firstUnderscore + 1);
var split = [string.substring(0, secondUnderscore),
string.substring(secondUnderscore)];
Paste it into your browser's console to try it out. No need for a jsFiddle.

var string = "this_is_my_string";
var splitChar = string.indexOf('_', string.indexOf('_') + 1);
var result = [string.substring(0, splitChar),
string.substring(splitChar, string.length)];
This should work.

var str = "this_is_my_string";
var matches = str.match(/(.*?_.*?)(_.*)/); // MAGIC HAPPENS HERE
var firstPart = matches[1]; // this_is
var secondPart = matches[2]; // _my_string
This uses regular expressions to find the first two underscores, and captures the part up to it and the part after it. The first subexpression, (.*?_.*?), says "any number of characters, an underscore, and again any number of characters, keeping the number of characters matched as small as possible, and capture it". The second one, (_.*) means "match an underscore, then any number of characters, as much of them as possible, and capture it". The result of the match function is an array starting with the full matched region, followed by the two captured groups.

I know this post is quite old... but couldn't help but notice that no one provided a working solution. Here's one that works:
String str = "this_is_my_string";
String undScore1 = str.split("_")[0];
String undScore2 = str.split("_")[1];
String bothUndScores = undScore1 + "_" + undScore2 + "_";
String allElse = str.split(bothUndScores)[1];
System.out.println(allElse);
This is assuming you know there will always be at least 2 underscores - "allElse" returns everything after the second occurrence.