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Rex match a string in JS

I have few set of strings as mentioned below
/v4/users/1
/v4/users/1/vehicles/1
/v4/users
/v4/users?page=1
I would like to get users in all four cases as output using regex in Javascript
I tried below in https://www.regextester.com/
(?<=/v4/).*.(?=/[^/]*/)
It doesn't seem to come up right.
Any help on this would be appreciated.

You were close with the positive lookbehind. This works:
'/v4/users/1/vehicles/1'.match(/(?<=\/v4\/)[^\/\?]*/)
This matches users because after the lookbehind you match everything until just before the next slash.

/\/v4\/(\w+)/g
This will put users in a capture group. If you want you can make it a named group as well.
You can try it here:
https://regex101.com/r/0OOr0g/1

Why would the replace with regex not work even though the regex does?

There may be a very simple answer to this, probably because of my familiarity (or possibly lack thereof) of the replace method and how it works with regex.
Let's say I have the following string: abcdefHellowxyz
I just want to strip the first six characters and the last four, to return Hello, using regex... Yes, I know there may be other ways, but I'm trying to explore the boundaries of what these methods are capable of doing...
Anyway, I've tinkered on http://regex101.com and got the following Regex worked out:
/^(.{6}).+(.{4})$/
Which seems to pass the string well and shows that abcdef is captured as group 1, and wxyz captured as group 2. But when I try to run the following:
"abcdefHellowxyz".replace(/^(.{6}).+(.{4})$/,"")
to replace those captured groups with "" I receive an empty string as my final output... Am I doing something wrong with this syntax? And if so, how does one correct it, keeping my original stance on wanting to use Regex in this manner...
Thanks so much everyone in advance...

The code below works well as you wish
"abcdefHellowxyz".replace(/^.{6}(.+).{4}$/,"$1")
I think that only use ()to capture the text you want, and in the second parameter of replace(), you can use $1 $2 ... to represent the group1 group2.
Also you can pass a function to the second parameter of replace,and transform the captured text to whatever you want in this function.
For more detail, as #Akxe recommend , you can find document on https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/replace.

You are replacing any substring that matches /^(.{6}).+(.{4})$/, with this line of code:
"abcdefHellowxyz".replace(/^(.{6}).+(.{4})$/,"")
The regex matches the whole string "abcdefHellowxyz"; thus, the whole string is replaced. Instead, if you are strictly stripping by the lengths of the extraneous substrings, you could simply use substring or substr.
Edit
The answer you're probably looking for is capturing the middle token, instead of the outer ones:
var str = "abcdefHellowxyz";
var matches = str.match(/^.{6}(.+).{4}$/);
str = matches[1]; // index 0 is entire match
console.log(str);

why this regexp returns match?

http://jsfiddle.net/sqee98xr/
var reg = /^(?!managed).+\.coffee$/
var match = '20150212214712-test-managed.coffee'.match(reg)
console.log(match) // prints '20150212214712-test-managed.coffee'
I want to match regexp only if there is not word "managed" present in a string - how I can do that?

Negative lookaheads are weird. You have to match more than just the word you are looking for. It's weird, I know.
var reg = /^(?!.*managed).+\.coffee$/
http://jsfiddle.net/sqee98xr/3/
EDIT: It seems I really got under some people's skin with the "weird" descriptor and lay description. It's weird because on a surface level the term "negative lookahead" implies "look ahead and make sure the stuff in these parenthesis isn't up there, then come back and continue matching". As a lover of regex, I still proclaim this naming is weird, especially to first time users of the assertion. To me it's easier to think of it as a "not" operator as opposed to something which actually crawls forward and "looks ahead". In order to get behavior to resemble an actual "look ahead", you have to match everything before the search term, hence the .*.
An even easier solution would have been to remove the start-of-string (^) assertion. Again, to me it's easier to read ?! as "not".
var reg = /(?!managed).+\.coffee$/

While #RyanWheale's solution is correct, the explanation isn't correct. The reason essentially is that a string that contains the word "managed" (such as "test-managed" ) can count as not "managed". To get an idea of this first lets look at the regular expression:
/^(?!managed).+\.coffee$/
// (Not "managed")(one or more characters)(".")("coffee")
So first we cannot have a string with the text "managed", then we can have one or more characters, then a dot, followed by the text "coffee". Here is an example that fulfills this.
"Hello.coffee" [ PASS ]
Makes sense, "Hello" certainly is not "managed". Here is another example that works from your string:
"20150212214712-test-managed.coffee" [ PASS ]
Why? Because "20150212214712-test-managed" is not the string "managed" even though it contains the string, the computer does not know that's what you mean. It thinks that "20150212214712-test-managed" as a string that isn't "managed" in the same way "andflaksfj" isn't "managed". So the only way it fails is if "managed" was at the start of the string:
"managed.coffee" [ FAIL ]
This isn't just because the text "managed" is there. Say the computer said that "managed." was not "managed". It would indeed pass the (?!managed) part but the rest of the string would just be coffee and it would fail because there is no ".".
Finally the solution to this is as suggested by the other answer:
/^(?!.*managed).+\.coffee$/
Now the string "20150212214712-test-managed.coffee" fails because no matter how it's looked at: "test-managed", "-managed", "st-managed", etc. Would still count as (?!.*managed) and fail. As in the example above this one it could try adding a sub-string from ".coffee", but as explained this would cause the string to fail in the rest of the regexp ( .+\.coffee$ ).
Hopefully this long explanation explained that Negative look-aheads are not weird, just takes your request very literally.

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

How to search csv string and return a match by using a Javascript regex

I'm trying to extract the first user-right from semicolon separated string which matches a pattern.
Users rights are stored in format:
LAA;LA_1;LA_2;LE_3;
String is empty if user does not have any rights.
My best solution so far is to use the following regex in regex.replace statement:
.*?;(LA_[^;]*)?.*
(The question mark at the end of group is for the purpose of matching the whole line in case user has not the right and replace it with empty string to signal that she doesn't have it.)
However, it doesn't work correctly in case the searched right is in the first position:
LA_1;LA_2;LE_3;
It is easy to fix it by just adding a semicolon at the beginning of line before regex replace but my question is, why doesn't the following regex match it?
.*?(?:(?:^|;)(LA_[^;]*))?.*
I have tried numerous other regular expressions to find the solution but so far without success.

I am not sure I get your question right, but in regards to the regular expressions you are using, you are overcomplicating them for no clear reason (at least not to me). You might want something like:
function getFirstRight(rights) {
var m = rights.match(/(^|;)(LA_[^;]*)/)
return m ? m[2] : "";
}

You could just split the string first:
function getFirstRight(rights)
{
return rights.split(";",1)[0] || "";
}

To answer the specific question "why doesn't the following regex match it?", one problem is the mix of this at the beginning:
.*?
eventually followed by:
^|;
Which might be like saying, skip over any extra characters until you reach either the start or a semicolon. But you can't skip over anything and then later arrive at the start (unless it involves newlines in a multiline string).
Something like this works:
.*?(\bLA_[^;]).*
Meaning, skip over characters until a word boundary followed by "LA_".