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How do i allow only one (dash or dot or underscore) in a user form input using regular expression in javascript?

I'm trying to implement a username form validation in javascript where the username
can't start with numbers
can't have whitespaces
can't have any symbols but only One dot or One underscore or One dash
example of a valid username: the_user-one.123
example of invalid username: 1----- user
i've been trying to implement this for awhile but i couldn't figure out how to have only one of each allowed symbol:-
const usernameValidation = /(?=^[\w.-]+$)^\D/g
console.log(usernameValidation.test('1username')) //false
console.log(usernameValidation.test('username-One')) //true

How about using a negative lookahead at the start:
^(?!\d|.*?([_.-]).*\1)[\w.-]+$
This will check if the string
neither starts with digit
nor contains two [_.-] by use of capture and backreference
See this demo at regex101 (more explanation on the right side)

Preface: Due to my severe carelessness, I assumed the context was usage of the HTML pattern attribute instead of JavaScript input validation. I leave this answer here for posterity in case anyone really wants to do this with regex.
Although regex does have functionality to represent a pattern occuring consecutively within a certain number of times (via {<lower-bound>,<upper-bound>}), I'm not aware of regex having "elegant" functionality to enforce a set of patterns each occuring within a range of number of times but in any order and with other patterns possibly in between.
Some workarounds I can think of:
Make a regex that allows for one of each permutation of ordering of special characters (note: newlines added for readability):
^(?:
(?:(?:(?:[A-Za-z][A-Za-z0-9]*\.?)|\.)[A-Za-z0-9]*-?[A-Za-z0-9]*_?)|
(?:(?:(?:[A-Za-z][A-Za-z0-9]*\.?)|\.)[A-Za-z0-9]*_?[A-Za-z0-9]*-?)|
(?:(?:(?:[A-Za-z][A-Za-z0-9]*-?)|-)[A-Za-z0-9]*\.?[A-Za-z0-9]*_?)|
(?:(?:(?:[A-Za-z][A-Za-z0-9]*-?)|-)[A-Za-z0-9]*_?[A-Za-z0-9]*\.?)|
(?:(?:(?:[A-Za-z][A-Za-z0-9]*_?)|_)[A-Za-z0-9]*\.?[A-Za-z0-9]*-?)|
(?:(?:(?:[A-Za-z][A-Za-z0-9]*_?)|_)[A-Za-z0-9]*-?[A-Za-z0-9]*\.?)
)[A-Za-z0-9]*$
Note that the above regex can be simplified if you don't want usernames to start with special characters either.
Friendly reminder to also make sure you use the HTML attributes to enforce a minimum and maximum input character length where appropriate.
If you feel that regex isn't well suited to your use-case, know that you can do custom validation logic using javascript, which gives you much more control and can be much more readable compared to regex, but may require more lines of code to implement. Seeing the regex above, I would personally seriously consider the custom javascript route.
Note: I find https://regex101.com/ very helpful in learning, writing, and testing regex. Make sure to set the "flavour" to "JavaScript" in your case.

I have to admit that Bobble bubble's solution is the better fit. Here ia a comparison of the different cases:
console.log("Comparison between mine and Bobble Bubble's solution:\n\nusername mine,BobbleBubble");
["valid-usrId1","1nvalidUsrId","An0therVal1d-One","inva-lid.userId","anot-her.one","test.-case"].forEach(u=>console.log(u.padEnd(20," "),chck(u)));
function chck(s){
return [!!s.match(/^[a-zA-Z][a-zA-Z0-9._-]*$/) && ( s.match(/[._-]/g) || []).length<2, // mine
!!s.match(/^(?!\d|.*?([_.-]).*\1)[\w.-]+$/)].join(","); // Bobble bulle
}
The differences can be seen in the last three test cases.

Get an example matched text from a regex pattern [duplicate]

Is there any way of generating random text which satisfies provided regular expression.
I am looking for a function which works like below
var reg = Some Regular Expression
var str = RandString(reg)

I have seen fairly good solutions in perl and ruby on github, but I think there are technical issues that make a complete solution impossible. For example, /[0-9]+/ has an infinite upper bound, which is not practical for selecting random numbers from.
Never seen it in JavaScript, but you could translate.
EDIT: After googling for a few seconds...
https://github.com/fent/randexp.js

if you know what the regular expression is, you can just generate random strings, then use a function that references the index of the letters and changes them as needed. Regex expressions vary widely, so it will be difficult to find one in particular that satisfies all possible regex.

Your question is pretty open so hopefully this steers you to the right solution. Get the current time (in seconds), MD5 it, check it against a REGEX, return the match.
Running Example: http://jsfiddle.net/MattLo/3gKrb/
Usage: RandString(/([A-Za-z])/ig); // expected to be a string

For JavaScript, the following modules can generate a random match to a regex:
pxeger
randexp.js
regexgen

RegEx that works in Javascript won't do so in PHP

I will try to make my question short yet understandable, I have a simple RegEx I use in javascript to check for characters that aren't alphanumeric (AKA Symbols). It would be "/[$-/:-?{-~!"^_`[]]/"
In javascript, doing
if(/[$-/:-?{-~!"^_`\[\]]/.test( string ))
just works, if any of those characters are in the string, it will give true, else, it will give false. I tried to do the same in PHP, the following way
if(preg_match('/[$-/:-?{-~!"^_`\[\]]/', $string ))
other regexes work when done this way, but this particular one simply will give false no matter what when ran in PHP.
Is there any reason to this? Am I doing something wrong? Does PHP comprehend regexes in a different way? What should I change to make it work?
Thanks for your time.

Since php uses PCRE, you will get a pattern error using delimiter / as seen here http://regex101.com/r/3ILGgE/1
So, it should be escaped correctly.
Using / as the delimiter, the string is
'/[$-\/:-?{-~!"^_`\[\]]/'
Using ~ as the delimiter, the string is
'~[$-/:-?{-\~!"^_`\[\]]~'
Also, be aware you have a couple of range's in the class $-/ and :-? and {-~
that will include the characters between the from/to range characters as well
and does not include the range character - itself as it is an operator.

Getting this regex expression to work in javascript

I have an html checkbox element with the following name:
type_config[selected_licenses][CC BY-NC-ND 3.0]
I would like to break this name apart as follows and returned as part of an array:
["type_config", "[selected_licenses]", "[CC BY-NC-ND 3.0]", "[selected_licenses][CC BY-NC-ND 3.0]"]
I thought I could do this by using a regular expression in javascript. Here is the expression that I am using:
matches = /([a-zA-Z0-9_]*)((\[[a-zA-Z0-9_\.\s]*\])+)*/.exec(element_name);
but this is the result I am getting in my matches variable:
["type_config[selected_licenses]", "type_config", "[selected_licenses]", "[selected_licenses]", index: 0, input: "type_config[selected_licenses][CC BY-ND 3.0]"]
I am half way there. What am I doing wrong in my regular expression? I guess I should also ask if it is possible to accomplish what I want with a regex?
Thanks.

The problem with this kind of goal is that there's no simple way to achieve this with regular expression, i.e. a simple match call. In short, even if you put a quantifier after a capturing group, the captured string will always be just one.
You'll have to rely on something more specific, like breaking the string with a repeated use of indexOf, or something like
name.split(/(?=\[)/);
Maybe you want to be sure that name is formally correct.

This is a very ugly problem. I don't know how repeatable this is, but I can do it:
Regex
^(\w+)(?<firstbracket>\[(?<secondbracket>[^]]*)\]\[(.*?)\])$
Replacement
["$1", "[$3]", "[$4]", "$2"]
Demo
http://regex101.com/r/eD9mH8

Regular expression for percentage

I have the following RegExp which verifies the value to be 100%
(^100([.]0{1,2})?)$|(^\\d{1,2}([.]\\d{1,2})?)$
I'm trying to get it to accept percentages without the leading zero, example
.5 rather than just 0.5.
If someone could help me to correct this expression so that it would accept both, I'd really appreciate it.

Here you should find your correct regular expression.

You can add ^([.]\d{1,2})?) as another possible string:
(^100([.]0{1,2})?)$|(^\d{1,2}([.]\d{1,2})?)$|(^([.]\d{1,2})?)$