Develop Reference

How to get next 3 characters after a substring if it exist inside a string?

I have this string that's. The &# substring is common but number after it changes in almost every object of my JSON data. So I want to detect if there is this substring, then get next three characters after it replace it with something else. How can I do it?

You can do it like this
var para = 'that's';
para = para.substr(para.indexOf('#')+1, 3);
syntax:
substr(start, length)
indexOf(searchvalue, [start])

Assuming you want to replace everything from &# until ; (if not, please update your question by specifying expected output):
You can use String.prototype.replace() with a regular expression:
var para = 'some string ' middle † end';
para = para.replace(/&#([\d]*);/g, 'replacement');
The g modifier is important to replace all occurences in the string.
With the RegEx used, you can include the found number (between &# and ;) in the replacement string by using $1.

you can define and use a utility function to replace HTML entities like that:
function decode(text, replaceWith = '') {
return text.replace(/&#(\d+);/g, replaceWith)
}

Javascript (Regex): How do i replace (Backslash + Double Quote) pairs?

In Javascript, i want the below original string:
I want to replace \"this\" and \"that\" words, but NOT the one "here"
.. to become like:
I want to replace ^this^ and ^that^ words, but NOT the one "here"
I tried something like:
var str = 'I want to replace \"this\" and \"that\" words, but NOT the one "here"';
str = str.replace(/\"/g,"^");
console.log( str );
Demo: JSFiddle here.
But still .. the output is:
I want to replace ^this^ and ^that^ words, but NOT the one ^here^
Which means i wanted to replace only the \" occurrences but NOT the " alone. But i cannot.
Please help.

As #adeneo's comment, your string was created wrong and not exactly like your expectation. Please try this:
var str = 'I want to replace \\"this\\" and \\"that\\" words, but not the one "here"';
str = str.replace(/\\\"/g,"^");
console.log(str);

You can use RegExp /(")/, String.prototype.lastIndexOf(), String.prototype.slice() to check if matched character is last or second to last match in input string. If true, return original match, else replace match with "^" character.
var str = `I want to replace \"this\" and \"that\" words, but NOT the one "here"`;
var res = str.replace(/(")/g, function(match, _, index) {
return index === str.lastIndexOf(match)
|| index === str.slice(0, str.lastIndexOf(match) -1)
.lastIndexOf(match)
? match
: "^"
});
console.log(res);

The problem with String.prototype.replace is that it only replaces the first occurrence without Regular Expression. To fix this, you need to add a g and the end of the RegEx, like so:
var mod = str => str.replace(/\\\"/g,'^');
mod('I want to replace \\"this\\" and \\"that\\" words, but NOT the one "here"');
A less effective but easier to understand to do what you wanted is to split the string with the delimiter and then join it with the replacement, like so:
var mod = str => str.split('\\"').join('^');
mod('I want to replace \\"this\\" and \\"that\\" words, but NOT the one "here"');
Note: You can wrap a string with either ' or ". Suppose your string contains ", i.e. a"a, you will need to put an \ in front of the " as "a"a" causes syntax error. 'a"a' won't cause syntax error as the parser knows the " is part of the string, but when you put a \ in front of " or any other special characters, it means the following character is a special character. So 'a\"a' === 'a"a' === "a\"a". If you want to store \, you will need to use \ regardless of the type of quote you use, so to store \", you will need to use '\\"', '\\\"' or "\\\"".

How to ignore escape characters in javascript?

I have the following string:
var str = '\x27';
I have no control on it, so I cannot write it as '\\x27' for example. Whenever I print it, i get:
'
since 27 is the apostrophe. When I call .length on it, it gives me 1. This is of course correct, but how can I treat it like a not escaped string and have it print literally
\x27
and give me a length of 4?

I'm not sure if you should do what you are trying to do, but this is how it works:
var s = '\x27';
var sEncoded = '\\x' + s.charCodeAt(0).toString(16);
s is a string that contains one character, the apostrophe. The character code as a hexadecimal number is 27.

After the assignment var str = '\x27';, you can't tell where the contents of str came from. There's no way to find out whether a string literal was assigned, or whether the string literal contained an escape sequence. All you have is a string containing a single apostrophe character (Unicode code point U+0027). The original assignment could have been
var str = '\x27'; // or
var str = "'"; // or
var str = String.fromCodePoint(3 * 13);
There's simply no way to tell.
That said, your question looks like an XY problem. Why are you trying to print \x27 in the first place?

Trimming String in Javascript

I have string like var a=""abcd""efgh"".How do I print the output as abcd""efgh by removing first and last double quote of a string I used a.replace(/["]/g,'') but it is removing all the double quotes of a string.How do i get the output as abcd""efgh.Suggest me an idea.

You can use
var a='"abcd""efgh"';
a.replace(/^"+|"+$/g, '');
From the comments here is the explanation
Explanation
There are 2 parts ^"+ and "+$ separated by | which is the regex equivalent of the or
^ is for starts-with and "+ is for one or more "
Similarly $ is for ends-with
The //g is for global replacement otherwise the only the first occurrence will be replaced

Try use this
var a='"abcd""efgh"';
a.replace(/^"|"$/g, '');

How to detect a series of characters in a string?

For example, I have a string:
"This is the ### example"
I would like to substring the ### out of the above string?
The number of Hash keys may vary, so I would like to find out and replace the ### pattern with, say, 001 for example.
Can anybody help?

You can also do a replace. I am familiar with the C# version of this,
string stringValue = "Thia is the ### example";
stringValue.Replace("###", "");
This would remove ### completely from the above string. Again you would have to know the exact string.
In JavaScript, it's similar - .replace (with a lowercase r) is used. So:
var stringValue = "This is the ### example";
var replacedValue = stringValue.replace('###', '');

You'll want to investigate either "Regular Expressions" for this, or, if you know the precise position and length of the characters you are interested in, you can simply use String's .substring method.

If you want to capture multiple # characters, then you'll need regular expressions:
var myString = "This is #### the example";
var result = myString.replace(/#+/g, '');
If you want to remove the space too, you can use the regex /#+\s|\s#+|#+/.

If the rest of the string is known, just get the part that you need:
var example = str.substr(12, str.length - 20);

The javascript match method will return an array of substrings matching a regular expression. You can use this to determine the number of matching characters to be replaced. Assuming you want to replace each octothorpe with a random digit, you could use code like this:
var exampleStr = "This is the ### example";
var swapThese = exampleStr.match(/#/g);
if (swapThese) {
for (var i=0;i<swapThese.length;i++) {
var swapThis = new RegExp(swapThese[i]);
exampleStr = exampleStr.replace(swapThis,Math.floor(Math.random()*9));
}
}
alert(exampleStr); // or whatever you want to do with it
Note that the code only loops the length of the array if it's present: if (swapThese) {
This check is necessary because if the match method finds no matches, it returns null rather than an empty array. Trying to iterate through null value will break.