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Regex to get the text between two characters?

I want to replace a text after a forward slash and before a end parantheses excluding the characters.
My text:
<h3>notThisText/IWantToReplaceThis)<h3>
$('h3').text($('h3').text().replace(regEx, 'textReplaced'));
Wanted result after replace:
notThisText/textReplaced)
I have tried
regex = /([^\/]+$)+/ //replaces the parantheses as well
regex = \/([^\)]+) //replaces the slash as well
but as you can see in my comments neither of these excludes both the slash and the end parantheses. Can someone help?

A pattern like /(?<=\/)[^)]+(?=\))/ won't work in JS as its regex engine does not support a lookbehind construct. So, you should use one of the following solutions:
s.replace(/(\/)[^)]+(\))/, '$1textReplaced$2')
s.replace(/(\/)[^)]+(?=\))/, '$1textReplaced')
s.replace(/(\/)[^)]+/, '$1textReplaced')
s.replace(/\/[^)]+\)/, '/textReplaced)')
The (...) forms a capturing group that can be referenced to with $ + number, a backreference, from the replacement pattern. The first solution is consuming / and ), and puts them into capturing groups. If you need to match consecutive, overlapping matches, use the second solution (s.replace(/(\/)[^)]+(?=\))/, '$1textReplaced')). If the ) is not required at the end, the third solution (replace(/(\/)[^)]+/, '$1textReplaced')) will do. The last solution (s.replace(/\/[^)]+\)/, '/textReplaced)')) will work if the / and ) are static values known beforehand.

You can use str.split('/')
var text = 'notThisText/IWantToReplaceThis';
var splited = text.split('/');
splited[1] = 'yourDesireText';
var output = splited.join('/');
console.log(output);

Try Following: In your case startChar='/', endChar = ')', origString=$('h3').text()
function customReplace(startChar, endChar, origString, replaceWith){
var strArray = origString.split(startChar);
return strArray[0] + startChar + replaceWith + endChar;
}

First of all, you didn't define clearly what is the format of the text which you want to replace and the non-replacement part. For example,
Does notThisText contain any slash /?
Does IWantToReplaceThis contain any parentheses )?
Since there are too many uncertainties, the answer here only shows up the pattern exactly matches your example:
yourText.replace(/(\/).*?(\))/g, '$1textReplaced$2')

var text = "notThisText/IWantToReplaceThis";
text = text.replace(/\/.*/, "/whatever");
output : "notThisText/whatever"`

Javascript nested square brackets in string

I am looking for an easier (and less hacky) way to get the substring of what is inside matching square brackets in a string. For example, lets say this is the string:
[ABC[D][E[FG]]HIJK[LMN]]OPQR[STUVW]XYZ
I want the substring:
ABC[D][E[FG]]HIJK[LMN]
Right now, I am looping through the string and counting the open and closed brackets, and when those numbers are the same, I take substring of the first open bracket and last closed bracket.
Is there an easier way to do this (ie with regex), so that I do need to loop through every character?

Here's another approach, an ugly hack which turns the input into a JS array representation and then parses it using JSON.parse:
function parse(str) {
return JSON.parse('[' +
str.split('') . join(',') . // insert commas
replace(/\[,/g, '[') . // clean up leading commas
replace(/,]/g, ']') . // clean up trailing commas
replace(/\w/g, '"$&"') // quote strings
+ ']');
}
>> hack('A[B]C')
<< ["A", ["B"], "C"]
Now a stringifier to turn arrays back into the bracketed form:
function stringify(array) {
return Array.isArray(array) ? '[' + array.map(stringify).join('') + ']' : array;
}
Now your problem can be solved by:
stringify(parse("[ABC[D][E[FG]]HIJK[LMN]]OPQR[STUVW]XYZ")[0])

Not sure if I get the question right (sorry about that).
So you mean that if you were to have a string of characters X, you would like to check if the string combination Y is contained within X?
Where Y being ABC[D][E[FG]]HIJK[LMN]
If so then you could simply do:
var str = "[ABC[D][E[FG]]HIJK[LMN]]OPQR[STUVW]XYZ";
var res = str.match(/ABC\[D]\[E\[FG]]HIJK\[LMN]/);
The above would then return the string literal Y as it matches what is inside str.
It is important that you pay attention to the fact that the symbols [ are being escaped with a \. This is because in regex if you were to have the two square brackets with any letter in between (ie. [asd]) regex would then match the single characters included in the specified set.
You can test the regex here:
https://regex101.com/r/zK3vZ3/1

I think the problem is to get all characters from an opening square bracket up to the corresponding closing square bracket. Balancing groups are not implemented in JavaScript, but there is a workaround: we can use several optional groups between these square brackets.
The following regex will match up to 3 nested [...] groups and you can add the capturing groups to support more:
\[[^\]\[]*(?:
\[[^\]\[]*(?:
\[[^\]\[]*(?:\[[^\]\[]*\])*\]
)*[^\]\[]*
\][^\]\[]*
)*[^\]\[]*
\]
See example here. However, performance may be not that high with such heavy backtracking.
UPDATE
Use XRegExp:
var str = '[ABC[D][E[FG]]HIJK[LMN]]OPQR[STUVW]XYZ';
// First match:
var res = XRegExp.matchRecursive(str, '\\[', ']');
document.body.innerHTML = "Getting the first match:<br/><pre>" + JSON.stringify(res, 0, 4) + "</pre><br/>And now, multiple matches (add \"g\" modifier when defining the XRegExp)";
// Multiple matches:
res = XRegExp.matchRecursive(str, '\\[', ']', 'g');
document.body.innerHTML += "<pre>" + JSON.stringify(res, 0, 4) + "</pre>";
<script src="https://cdnjs.cloudflare.com/ajax/libs/xregexp/2.0.0/xregexp-all-min.js"></script>

How can I get a substring located between 2 quotes?

I have a string that looks like this: "the word you need is 'hello' ".
What's the best way to put 'hello' (but without the quotes) into a javascript variable? I imagine that the way to do this is with regex (which I know very little about) ?
Any help appreciated!

Use match():
> var s = "the word you need is 'hello' ";
> s.match(/'([^']+)'/)[1];
"hello"
This will match a starting ', followed by anything except ', and then the closing ', storing everything in between in the first captured group.

http://jsfiddle.net/Bbh6P/
var mystring = "the word you need is 'hello'"
var matches = mystring.match(/\'(.*?)\'/); //returns array
​alert(matches[1]);​

If you want to avoid regular expressions then you can use .split("'") to split the string at single quotes , then use jquery.map() to return just the odd indexed substrings, ie. an array of all single-quoted substrings.
var str = "the word you need is 'hello'";
var singleQuoted = $.map(str.split("'"), function(substr, i) {
return (i % 2) ? substr : null;
});
DEMO
CAUTION
This and other methods will get it wrong if one or more apostrophes (same as single quote) appear in the original string.

Javascript Regex- replace sequence of characters with same number of another character

I'm trying to replace part of a string with the same number of dummy characters in JavaScript, for example: '==Hello==' with '==~~~~~=='.
This question has been answered using Perl and PHP, but I can't get it to work in JavaScript. I've been trying this:
txt=txt.replace(/(==)([^=]+)(==)/g, "$1"+Array("$2".length + 1).join('~')+"$3");
The pattern match works fine, but the replacement does not - the second part adds '~~' instead of the length of the pattern match. Putting the "$2" inside the parentheses doesn't work. What can I do to make it insert the right number of characters?

Use a function for replacement instead:
var txt = "==Hello==";
txt = txt.replace(/(==)([^=]+)(==)/g, function ($0, $1, $2, $3) {
return $1 + (new Array($2.length + 1).join("~")) + $3;
});
alert(txt);
//-> "==~~~~~=="

The issue with the expression
txt.replace(/(==)([^=]+)(==)/g, "$1"+Array("$2".length + 1).join('~')+"$3")
is that "$2".length forces $2 to be taken as a string literal, namely the string "$2", that has length 2.
From the MDN docs:
Because we want to further transform the result of the match before the final substitution is made, we must use a function.
This forces evaluation of the match before the transformation.
With an inline function as parameter (and repeat) -- here $1, $2, $3 are local variables:
txt.replace(/(==)([^=]+)(==)/g, (_,$1,$2,$3) => $1+'~'.repeat($2.length)+$3);
txt = '==Hello==';
//inline function
console.log(
txt.replace(/(==)([^=]+)(==)/g, (_, g1, g2, g3) => g1 + '~'.repeat(g2.length) + g3)
);

The length attribute is being evaluated before the $2 substitution so replace() won't work. The function call suggested by Augustus should work, another approach would be using match() instead of replace().
Using match() without the /g, returns an array of match results which can be joined as you expect.
txt="==Hello==";
mat=txt.match(/(==)([^=]+)(==)/); // mat is now ["==Hello==","==","Hello","=="]
txt=mat[1]+Array(mat[2].length+1).join("~")+mat[3]; // txt is now "==~~~~~=="
You excluded the leading/trailing character from the middle expression, but if you want more flexibility you could use this and handle anything bracketed by the leading/trailing literals.
mat=txt.match(/(^==)(.+)(==$)/)

A working sample uses the following fragment:
var processed = original.replace(/(==)([^=]+)(==)/g, function(all, before, gone, after){
return before+Array(gone.length+1).join('~')+after;
});
The problem in your code was that you always measured the length of "$2" (always a string with two characters). By having the function you can measure the length of the matched part. See the documentation on replace for further examples.

Javascript RegEx non-capturing prefix

I am trying to do some string replacement with RegEx in Javascript. The scenario is a single line string containing long comma-delimited list of numbers, in which duplicates are possible.
An example string is: 272,2725,2726,272,2727,297,272 (The end may or may not end in a comma)
In this example, I am trying to match each occurrence of the whole number 272. (3 matches expected)
The example regex I'm trying to use is: (?:^|,)272(?=$|,)
The problem I am having is that the second and third matches are including the leading comma, which I do not want. I am confused because I thought (?:^|,) would match, but not capture. Can someone shed light on this for me? An interesting bit is that the trailing comma is excluded from the result, which is what I want.
For what it is worth, if I were using C# there is syntax for prefix matching that does what I want: (?<=^|,)
However, it appears to be unsupported in JavaScript.
Lastly, I know I could workaround it using string splitting, array manipulation and rejoining, but I want to learn.

Use word boundaries instead:
\b272\b
ensures that only 272 matches, but not 2725.
(?:...) matches and doesn't capture - but whatever it matches will be part of the overall match.
A lookaround assertion like (?=...) is different: It only checks if it is possible (or impossible) to match the enclosed regex at the current point, but it doesn't add to the overall match.

Here is a way to create a JavaScript look behind that has worked in all cases I needed.
This is an example. One can do many more complex and flexible things.
The main point here is that in some cases,
it is possible to create a RegExp non-capturing prefix
(look behind) construct in JavaScript .
This example is designed to extract all fields that are surrounded by braces '{...}'.
The braces are not returned with the field.
This is just an example to show the idea at work not necessarily a prelude to an application.
function testGetSingleRepeatedCharacterInBraces()
{
var leadingHtmlSpaces = ' ' ;
// The '(?:\b|\B(?={))' acts as a prefix non-capturing group.
// That is, this works (?:\b|\B(?=WhateverYouLike))
var regex = /(?:\b|\B(?={))(([0-9a-zA-Z_])\2{4})(?=})/g ;
var string = '' ;
string = 'Message has no fields' ;
document.write( 'String => "' + string
+ '"<br>' + leadingHtmlSpaces + 'fields => '
+ getMatchingFields( string, regex )
+ '<br>' ) ;
string = '{LLLLL}Message {11111}{22222} {ffffff}abc def{EEEEE} {_____} {4444} {666666} {55555}' ;
document.write( 'String => "' + string
+ '"<br>' + leadingHtmlSpaces + 'fields => '
+ getMatchingFields( string, regex )
+ '<br>' ) ;
} ;
function getMatchingFields( stringToSearch, regex )
{
var matches = stringToSearch.match( regex ) ;
return matches ? matches : [] ;
} ;
Output:
String => "Message has no fields"
fields =>
String => "{LLLLL}Message {11111}{22222} {ffffff}abc def{EEEEE} {_____} {4444} {666666} {55555}"
fields => LLLLL,11111,22222,EEEEE,_____,55555