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Mask a portion of a String using RegExp

I'm trying to mask a portion of a string using JavaScript.
e.g. Mask second and third segment of credit-card number like this using regex:
4567 6365 7987 3783 → 4567 **** **** 3783
3457 732837 82372 → 3457 ****** 82372
I just want to keep the first 4 numbers and the last 5 characters.
This is my first attempt: /(?!^.*)[^a-zA-Z\s](?=.{5})/g
https://regex101.com/r/ZBi54c/2

You can try this:
var cardnumber = '4567 6365 7987 3783';
var first4 = cardnumber.substring(0, 4);
var last5 = cardnumber.substring(cardnumber.length - 5);
mask = cardnumber.substring(4, cardnumber.length - 5).replace(/\d/g,"*");
console.log(first4 + mask + last5);

You could slice the first four digits and apply a replacement for the rest.
console.log(
['4567 6365 7987 3783', '3457 732837 82372'].map(
s => s.slice(0, 4) + s.slice(4).replace(/\d(?=.* )/g, '*')
)
);

The answer apparently satisfies the OP. Here is another solution using only Regexes:
function starry(match, gr1, gr2, gr3) {
var stars = gr2.replace(/\d/g, '*');
return gr1 + " " + stars + " " + gr3;
}
function ccStarry(str) {
var rex = /(\d{4})\s(\d{4}\s\d{4}|\d{6})\s(\d{4}|\d{5})/;
if (rex.test(str))
return str.replace(rex, starry);
else return "";
}
var s1 = "4567 6365 7987 3783";
var s2 = "3457 732837 82372";
var s3 = "dfdfdf";
console.log(ccStarry(s1));
console.log(ccStarry(s2));
console.log(ccStarry(s3));
This ensures that the pattern matches before trying any replacements. For example, in the third test case, it returns an empty string. The pattern can be updated to match other credit card patterns besides the ones given in the question.

I would like to elaborate more on the answer from #Nina Scholz, I use .slice() in the following sample code for masking the variable in 2 condition.
Just a simple variable var n = '12345567890'
Array object
// Single number
var n = '601115558888';
var singleNumber = n.slice(0, 4) + n.slice(4, n.length -4).replace(/\d/g,'*') + n.slice(n.length -4);
console.log(singleNumber);
// array of object
var obj = [{
contacts_name: 'Jason',
contacts_num : '651231239991'
},
{
contacts_name: 'King',
contacts_num : '60101233321'
}];
// Mask for the middle number, showing the first4 number and last4 number
// and replace the rest number with *
var num = obj.map((element, index) =>
element.contacts_num.slice(0,4)
+ element.contacts_num.slice(4, element.contacts_num.length-4).replace(/\d/g, '*')
+ element.contacts_num.slice(element.contacts_num.length -4)
);
console.log(num);

If it's JavaScript doing the regex masking, you've already failed because JS should never need to know the original card number, except when you've just received it from the user and are sending it to the server for the first time, in which case you shouldn't be masking it anyway so the user can check for typos.
I can't really help you there, you've already failed in the worst way.
Server-side, if the number is already broken into spaces*, then one option is: (in PHP but the same idea applies to all)
$parts = explode(" ",$fullnumber);
$first = array_shift($parts);
$last = array_pop($parts);
$middle = implode(" ",$parts);
$mask = preg_replace("/\d/","*",$middle);
$result = "$first $mask $last";
* it shouldn't be

Why can't println(); be used as a variable?

I'm fairly new to Javascript, and am confused on something. Why can't the command "println("..."); be called as a variable such as: var num = println("...");. I could be wrong, and if you are able to, I'd be happy to know how. But after some testing it seems like I can't. My test code is:
function start() {
var SENTINEL = "1 1";
var rollOne = Randomizer.nextInt(1, 6);
var rollTwo = Randomizer.nextInt(1, 6);
var num = println(rollOne + rollTwo);
if(num == SENTINEL) {
println("You did it");
}
}
All it's supposed to do is give to random numbers in a # # form and, if it sees that the numbers are 1,1, it will give a message. It wont give the message and can't seem to view the variable "num" as an actual variable. But when I change the variable num to simply asking the user for a number:
function start() {
var SENTINEL = -1;
var rollOne = Randomizer.nextInt(1, 6);
var rollTwo = Randomizer.nextInt(1, 6);
var num = readInt("Enter number");
if(num == SENTINEL) {
println("You did it");
}
}
And type in -1, it triggers the sentinel, thus promptly displaying the message. This is a really roundabout way to ask a simple question but I hope I can get some help. Thank you :)

Why can't the command "println("..."); be called as a variable such as: var num = println("...");
[...] It wont give the message and can't seem to view the variable
If the value returned is unusable, it is most likely undefined; i.e. The function println doesn't explicitly return anything.
In your case, you could try something like this:
var printInt = function(num) { println(num); return num; }
Note, println isn't part of the standard JavaScript language. For modern web browsers, it can be adapted to use (console.log(...)).
var printInt = function(num) { console.log(num); return num; }
And then to adapt to your code:
var num = printInt(rollOne + rollTwo);
But this still won't validate because you're comparing against "1 1" whereas your logic will return 2. JavaScript (as well as many other languages) implicitly uses addition when supplied with two numbers, but concatenation when supplied with at least one string.
var SENTINEL = "1 1"; // <---- String!
var SENTINEL = -1; // <---- Number!
So you should consider something like this instead (renamed accordingly):
var printRolls = function(text) { println(text); return text; }
var rolls = printRolls(rollOne + " " + rollTwo);
if(rolls == SENTINEL) {
println("You did it");
}
Or to simplify it a bit:
if(printRolls(rollOne + " " + rollTwo) == SENTINEL)
println("You did it");

It is possible that println doesn't return the string that is passed into. In that case, you can use
if (SENTINEL === rollOne + " " + rollTwo)
to format the string and properly test equality.

In JavaScript it is possible to assign the return value from any function to a variable similar to how you've done it:
var anyVariable = anyFunction();
But, some functions return the value undefined. Or they return a number, or an array, or...whatever.
I imagine your println() function prints the value you pass to it somewhere (on the screen? to the console?) and then returns undefined. Or if it is returning the printed value it is in a format different to what you have used in your SENTINEL variable. So then when you try to compare that with SENTINEL it won't be equal.
To fix your original function, assign the sum of the rolls to a variable, then print and test that:
function start() {
var SENTINEL = 2;
var rollOne = Randomizer.nextInt(1, 6);
var rollTwo = Randomizer.nextInt(1, 6);
var num = rollOne + rollTwo;
println(num);
if(num == SENTINEL) {
println("You did it");
}
}
EDIT: if you want the println() to display a string like "1 1" or "3 5" to show what each of the two rolls were then do this:
println(rollOne + " " + rollTwo);
That is, create a new string that is the result of concatenating rollOne's value with a single space and then rollTwo's value.

GET item from brackets in array javascript

I need some help. Here I have a string.
n[0] = '3(10)';
The task is to get only 10 from brackets. How to do it in javascript?

You can solve this with Regex :
This will do :
var a= '3(10)'.match(/\((.*?)\)/)
alert(a[1]) ;//10
The captured group will appear in the second index of the array (1)
Regarding your other comment/question :
I have a[0] = '3(10,5) 7(9,4)'; 10 and 9 are chances the task is to
get the number (3 or 7) with a bigger chance (10)
var finalNumber=-1;
var finalChance=-1;
var a = '3(10,5) 7(9,4)';
var m=a.match(/(\d+?)\((\d+?)\,/g);
for (var i=0;i<m.length;i++)
{
var number=m[i].match(/(\d+)\(/)[1]
var chance=m[i].match(/\((\d+)\,/)[1]
if (+chance>+finalChance)
{
finalChance=chance;
finalNumber=number;
}
}
console.log(finalNumber)
Jsbin

Use split() function to split your string with brackets two times :
var first_split = n[0].split(')')[0]; //first_split will return "3(10"
var result = first_split.split('(')[1]; //second split will return "10";
//To reduce the code you can do it in 1 line like this
var result = n[0].split(')')[0].split('(')[1]; // result = "10"

Insert space after certain character into javascript string

I'm working on a form and I'd like to mask the input of the phone numbers. The plugins what I found aren't okay for me since the area code could be 1 or 2 character long.
What I'd like to do is the following:
when the user types his number after the first two character the script inserts a space on keyup, then after the next three and later after every fourth character.
So when someone types 44444444444 then in the textbox appears 44 44 444 4444.
I must check the second group as well, and when someone types there for example 1, the the number must look like: 44 1 444 4444
Is any solution to do that?

You could do something like this:
http://jsfiddle.net/ffwAA/4/
Which applies this function to the string to get the desired formatting:
function formatCode(str){
var result = str;
str = str.replace(/\D+/g, "");
var m = str.match(/^(\d\d)(?:([2-90]\d|1)(?:(\d\d\d)(\d+)?)?)?$/);
if(m){
result = m[1] + " ";
if(m[2]) result += m[2] + " ";
if(m[3]) result += m[3] + " ";
if(m[4]){
result += m[4].split(/(\d{4})/).join(" ");
result = result.replace(/\s+/g, " ");
}
}
return result;
}
And using this jQuery to set it up:
function update(obj){
var val = obj.value;
var got = formatCode(val);
if(got != val)
obj.value = got;
}
var timer;
var prev_val = "";
$('#code').keyup(function(){
clearTimeout(timer);
// when adding numbers at the end of input, update at once
// don't want to update when editing in the middle of the string or removing parts of it
// because it would move the carret location to the end of input, and make it unusable
if(this.value.indexOf(prev_val) == 0){
update(this);
prev_val = this.value;
return;
}
prev_val = this.value;
// in other cases update 1 second after the changes are done
timer = setTimeout(update, 1000, this);
});

Have you tried the maskedInput plugin?
http://digitalbush.com/projects/masked-input-plugin/
I think it can solve your problem.
Hope this helps. Cheers

What is the most optimized or simplest way to reduce a file name in javascript

I recently created a function in javascript that takes in a file name and a max character limit where the result needs to follow these rules:
Always include file extension
If shrinking occurs, leave the first part and last part of the file name intact.
Always replace the removed characters with '...'
If file length is under the max then do nothing
You can assume the max is a least 5 chars long
Now I've already solved this, but it got me thinking if there is a more elegant or simple way to do this in javascript using regular expressions or some other technique. It also gave me an opportunity to try out jsFiddle. So with that in mind here is my function:
function ReduceFileName(name, max){
if(name.length > max){
var end = name.substring(name.lastIndexOf('.'));
var begin = name.substring(0, name.lastIndexOf('.'));
max = max - end.length - 3;
begin = begin.substr(0,max/2) + '...' + begin.substr(begin.length-(max/2) , max/2 + 1);
return begin + end;
}
return name;
}
And here it is on js Fiddle with tests

I'm not sure that regular expressions will be necessarily more elegant, but so far I came up with the following which passes your tests:
function ReduceFileName(name, max){
if(name.length > max) {
var ell ="\u2026"; // defines replacement characters
var ext = (/\.[^\.]*$/.exec(name) || [""])[0]; // gets extension (with dot) or "" if no dot
var m = (max-ell.length-ext.length)/2; // splits the remaining # of characters
var a = Math.ceil(m);
var z = Math.floor(m);
var regex = new RegExp("^(.{"+a+"}).*(.{"+z+"})"+ext, "");
var ret = regex.exec(name);
return ret[1]+ell+ret[2]+ext;
}
return name;
}

Since I didn't get much activity on this, I'm assuming there isn't a much better way to do this, so I'll consider my method as the answer until someone else comes up with something else.
function ReduceFileName(name, max){
if(name.length > max){
var end = name.substring(name.lastIndexOf('.'));
var begin = name.substring(0, name.lastIndexOf('.'));
max = max - end.length - 3;
begin = begin.substr(0,max/2) + '...' + begin.substr(begin.length-(max/2) , max/2 + 1);
return begin + end;
}
return name;
}