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Replace multiple characters by one character with regex

I have this string :
var str = '#this #is____#a test###__'
I want to replace all the character (#,_) by (#) , so the excepted output is :
'#this #is#a test#'
Note :
I did not knew How much sequence of (#) or (_) in the string
what I try :
I try to write :
var str = '#this #is__ __#a test###__'
str = str.replace(/[#_]/g,'#')
alert(str)
But the output was :
#this #is## ###a test#####
my try online
I try to use the (*) for sequence But did not work :
var str = '#this #is__ __#a test###__'
str = str.replace(/[#_]*/g,'#')
alert(str)
so How I can get my excepted output ?

A well written RegEx can handle your problem rather easily.
Quoting Mohit's answer to have a startpoint:
var str = '#this #is__ __#a test###__';
var formattedStr = str.replace(/[#_,]+/g, '#');
console.log( formattedStr );
Line 2:
Put in formattedStr the result of the replace method on str.
How does replace work? The first parameter is a string or a RegEx.
Note: RegExps in Javascripts are Objects of type RegExp, not strings. So writing
/yourRegex/
or
New RegExp('/yourRegex/')
is equivalent syntax.
Now let's discuss this particular RegEx itself.
The leading and trailing slashes are used to surround the pattern, and the g at the end means "globally" - do not stop after the first match.
The square parentheses describe a set of characters who can be supplied to match the pattern, while the + sign means "1 or more of this group".
Basically, ### will match, but also # or #####_# will, because _ and # belong to the same set.
A preferred behavior would be given by using (#|_)+
This means "# or _, then, once found one, keep looking forward for more or the chosen pattern".
So ___ would match, as well as #### would, but __## would be 2 distinct match groups (the former being __, the latter ##).
Another problem is not knowing wheter to replace the pattern found with a _ or a #.
Luckily, using parentheses allows us to use a thing called capturing groups. You basically can store any pattern you found in temporary variabiles, that can be used in the replace pattern.
Invoking them is easy, propend $ to the position of the matched (pattern).
/(foo)textnotgetting(bar)captured(baz)/ for example would fill the capturing groups "variables" this way:
$1 = foo
$2 = bar
$3 = baz
In our case, we want to replace 1+ characters with the first occurrence only, and the + sign is not included in the parentheses!
So we can simply
str.replace("/(#|_)+/g", "$1");
In order to make it work.
Have a nice day!

Your regex replaces single instance of any matched character with character that you specified i.e. #. You need to add modifier + to tell it that any number of consecutive matching characters (_,#) should be replaced instead of each character individually. + modifier means that 1 or more occurrences of specified pattern is matched in one go. You can read more about modifiers from this page:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp
var str = '#this #is__ __#a test###__';
var formattedStr = str.replace(/[#_,]+/g, '#');
console.log( formattedStr );

You should use the + to match one-or-more occurrences of the previous group.
var str = '#this #is__ __#a test###__'
str = str.replace(/[#_]+/g,'#')
alert(str)

Regex to add a new line break after each bracket

I have tried using replace with regex (/\([(]\)/g,'\1\n') but its not helping. Any help?

Regardless what the language is, a round bracket matching pattern is either \( or [(]. If you need to use a whole match value in the replacement, there are $& or $0 backreferences.
Thus, search for /[(]/g (you may add more chars into the character class, like [()\][]...) and replace with "$&\n" (or "$0\n").
See the regex demo.
A JS demo:
var regex = /[(]/g;
var str = "before(after and before 1(after1";
var subst = "$&\n";
var result = str.replace(regex, subst);
console.log(result);

Match parts of code

I'm trying to match parts of code with regex. How can I match var, a, =, 2 and ; from
"var a = 2;"
?

I believe you want this regexp: /\S+/g
To break it down: \S selects all non-whitespace characters, + makes sure you it selects multiple non whitespace characters together (i.e. 'var'),
and the 'g' flag makes sure it selects all of the occurrences in the string, and instead of stopping at the first one which is the default behavior.
This is a helpful link for playing around until you find the right regexp: https://regex101.com/#javascript

var str = "var a = 2;";
// clean the duplicate whitespaces
var no_duplicate_whitespace = str.replace(new RegExp("\\s+", "g"), " ");
// and split by space
var tokens = no_duplicate_whitespace.split(" ");
Or as #kuujinbo pointed out:
str.split(/\s+/);

Regex replace linebreak and comma

i've a question about regex, i've a text and it looks like below :
car,model,serie
,Mercedes,324,1,
,BMW,23423,1,
,OPEL,54322,1,
it should look like:
car,model,serie
Mercedes,324,1,
BMW,23423,1,
OPEL,54322,1,
so without commas at the beginning of the text.
What i tried :
var str2 = str.replace(/\n|\r/g, "");
but somehow, i couldn't add comma in regex.
can anyone help me?
Thanks in advance.

There have been a lot of responses to this question and for a newbie to regex it is probably a bit overwelming,
Overall the best response has been:
var str2 = str.replace(/^,/gm, '');
This works by using ^, to check if the first character is a comma and if it is, remove it. It also uses the g and m flags to do this for the first character of every line.
If you are curious about the other versions then read on:
1:
var str2 = str.replace(/^,+/gm, '');
This is a slight variant in that it will remove multiple consecutive commas at the beginning of each line, but based off of your dataset this is not required.
2:
var str2 = str.replace(/\n,/g, '\n');
This version works exactly the same as the first, however it finds each newline follow by a comma with \n, and replaces it with another newline.
3:
var str2 = str.replace(/(\n|\r),/g, '$1')
This version is the same as the previous however it doesn't make the assumption that the newline is a \n, it instead captures any newlines or carriage returns, it works the same as the m flag and ^,.
4:
var str2 = str.replace(/\n+|\r+|,+/g,"\n")
And finally there is this, this is a combination of all the previous regex's, it makes the assumption that you may have a lot mixed newlines and commas without any text, and that you would want to remove all of those characters, it is unnecessary for your examples.

Use this syntax:
str.replace(/^,/gm, '');

You can just use multiline flag and replace leading commas:
str = str.replace(/^,+/gm);
RegEx Demo

Try:
var str2 = str.replace(/(\n|\r),/g, '$1')
Your comma was actually placed outside the regex pattern, so you weren't far off :)

Replace all whitespace characters

I want to replace all occurrences of white space characters (space, tab, newline) in JavaScript.
How to do so?
I tried:
str.replace(/ /gi, "X")

You want \s
Matches a single white space
character, including space, tab, form
feed, line feed.
Equivalent to
[ \f\n\r\t\v\u00a0\u1680\u2000-\u200a\u2028\u2029\u202f\u205f\u3000\ufeff]
in Firefox and [ \f\n\r\t\v] in IE.
str = str.replace(/\s/g, "X");

We can also use this if we want to change all multiple joined blank spaces with a single character:
str.replace(/\s+/g,'X');
See it in action here: https://regex101.com/r/d9d53G/1
Explanation
/ \s+ / g
\s+ matches any whitespace character (equal to [\r\n\t\f\v ])
+ Quantifier — Matches between one and unlimited times, as many times as possible, giving back as needed (greedy)
Global pattern flags
g modifier: global. All matches (don't return after first match)

\s is a meta character that covers all white space. You don't need to make it case-insensitive — white space doesn't have case.
str.replace(/\s/g, "X")

Have you tried the \s?
str.replace(/\s/g, "X");

If you use
str.replace(/\s/g, "");
it replaces all whitespaces. For example:
var str = "hello my world";
str.replace(/\s/g, "") //the result will be "hellomyworld"

Try this:
str.replace(/\s/g, "X")

Not /gi but /g
var fname = "My Family File.jpg"
fname = fname.replace(/ /g,"_");
console.log(fname);
gives
"My_Family_File.jpg"

You could use the function trim
let str = ' Hello World ';
alert (str.trim());
All the front and back spaces around Hello World would be removed.

Actually it has been worked but
just try this.
take the value /\s/g into a string variable like
String a = /\s/g;
str = str.replaceAll(a,"X");

I've used the "slugify" method from underscore.string and it worked like a charm:
https://github.com/epeli/underscore.string#slugifystring--string
The cool thing is that you can really just import this method, don't need to import the entire library.