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jQuery Regex from String

I have the following Regex that comes from a data Attribute on an HTML Element:
/^$|^[0-9]{2}.[0-9]{4}$/g
When I (manually) do:
/^$|^[0-9]{2}.[0-9]{4}$/g.test('01.2012');
It works and returns true.
When I put the Regex in a Variable like so:
var inputRegex = $(this).attr('data-validation');
And do:
inputRegex.test(input);
I get:
inputRegex.test is not a function.
I know that this is because inputRegex is a String and String does not have a test function, but when I create a RegExp object (new RegExp($(this).attr('data-validation')) it breaks my Regular Expression by escaping:
/\/^$|^[0-9]{2}.[0-9]{4}$\/g/
How can I use the data-attribute value as a Regular Expression? Please note that I cannot do: var regex = new RegExp(string, 'g'); because the Regular Expression(s) come predefined from the attribute.

var pattern = '\d+';
var regExp = new RegExp(pattern, 'g');
'1234dasf13241234'.match(regExp)
is it what you need?
var pattern = $(this).attr('data-validation');;
var regExp = new RegExp(pattern, 'g');
regExp.test(input);
in your case

your problem is that you need to retrieve the regex pattern from a attribute of an element, but it is returning string, and you want the string value to be like inline on your javascript code, like declaring plainly a regex. If this is really what you want to achieve, the closest solution is to use eval function, see updated code below:
var stringreg = "var inputRegex =" + $("#test").attr('data-validation') + ";"
eval(stringreg);
inputRegex.test(input);

This could help
var validation = "/^$|^[0-9]{2}.[0-9]{4}$/g"; //$(this).attr('data-validation')
var startIndex = validation.indexOf('/')+1
var lastIndex = validation.lastIndexOf('/');
var pattern = validation.substring(startIndex,lastIndex);
var options = validation.substring(lastIndex+1);
var regExp = new RegExp(pattern, options);
regExp.test('01.2012');
// true

Javascript Regex as a string not working [duplicate]

This question already has answers here:
JavaScript RegExp objects
(5 answers)
Closed 7 years ago.
I am trying to understand why writing regex as a string is not working but using it without string does work.
this is my example:
var patt = new RegExp("/test_.*/gi");
var res = patt.test("test_4");
console.log(res);
will return false
but this:
var patt = /test_.*/gi;
var res = patt.test("test_4");
console.log(res);
will return true
what is the difference

Your syntax of RegExp is wrong.
The delimiters are not required when you use RegExp constructor to create new Regular Expression.
The flags should be passed as second parameter to the RegExp constructor.
And . should to be escaped, if you want to match . literal.
Usage
var patt = new RegExp("test_.*", "gi");
Demo
var patt = new RegExp("test_.*", "gi");
var res = patt.test("test_4");
document.write(res);

The regexp constructor does not need delimiters, also, flags are isolated in another argument which simplifies it to
var patt = new RegExp('test_.*', 'gi');
var res = patt.test("test_4");
console.log(res);

You do not need to include the / at the start and end of the regular expression when using the constructor and the flags need to be in the second argument. See the MDN Documentation for RegExp.
var patt = new RegExp( "test_.*", "gi" );
var res = patt.test("test_4");
console.log(res);

You are not calling the constructor correctly, the falgs should be passed as a second parameter:
new RegExp('test_.*', 'gi');
As you can see in Documentation, it should respect this format:
new RegExp(pattern[, flags])
This is your demo:
var patt = new RegExp("test_.*", 'gi');
var res = patt.test("test_4");
alert(res);

Use dynamic (variable) string as regex pattern in JavaScript

I want to add a (variable) tag to values with regex, the pattern works fine with PHP but I have troubles implementing it into JavaScript.
The pattern is (value is the variable):
/(?!(?:[^<]+>|[^>]+<\/a>))\b(value)\b/is
I escaped the backslashes:
var str = $("#div").html();
var regex = "/(?!(?:[^<]+>|[^>]+<\\/a>))\\b(" + value + ")\\b/is";
$("#div").html(str.replace(regex, "" + value + ""));
But this seem not to be right, I logged the pattern and its exactly what it should be.
Any ideas?

To create the regex from a string, you have to use JavaScript's RegExp object.
If you also want to match/replace more than one time, then you must add the g (global match) flag. Here's an example:
var stringToGoIntoTheRegex = "abc";
var regex = new RegExp("#" + stringToGoIntoTheRegex + "#", "g");
// at this point, the line above is the same as: var regex = /#abc#/g;
var input = "Hello this is #abc# some #abc# stuff.";
var output = input.replace(regex, "!!");
alert(output); // Hello this is !! some !! stuff.
JSFiddle demo here.
In the general case, escape the string before using as regex:
Not every string is a valid regex, though: there are some speciall characters, like ( or [. To work around this issue, simply escape the string before turning it into a regex. A utility function for that goes in the sample below:
function escapeRegExp(stringToGoIntoTheRegex) {
return stringToGoIntoTheRegex.replace(/[-\/\\^$*+?.()|[\]{}]/g, '\\$&');
}
var stringToGoIntoTheRegex = escapeRegExp("abc"); // this is the only change from above
var regex = new RegExp("#" + stringToGoIntoTheRegex + "#", "g");
// at this point, the line above is the same as: var regex = /#abc#/g;
var input = "Hello this is #abc# some #abc# stuff.";
var output = input.replace(regex, "!!");
alert(output); // Hello this is !! some !! stuff.
JSFiddle demo here.
Note: the regex in the question uses the s modifier, which didn't exist at the time of the question, but does exist -- a s (dotall) flag/modifier in JavaScript -- today.

If you are trying to use a variable value in the expression, you must use the RegExp "constructor".
var regex = "(?!(?:[^<]+>|[^>]+<\/a>))\b(" + value + ")\b";
new RegExp(regex, "is")

I found I had to double slash the \b to get it working. For example to remove "1x" words from a string using a variable, I needed to use:
str = "1x";
var regex = new RegExp("\\b"+str+"\\b","g"); // same as inv.replace(/\b1x\b/g, "")
inv=inv.replace(regex, "");

You don't need the " to define a regular expression so just:
var regex = /(?!(?:[^<]+>|[^>]+<\/a>))\b(value)\b/is; // this is valid syntax
If value is a variable and you want a dynamic regular expression then you can't use this notation; use the alternative notation.
String.replace also accepts strings as input, so you can do "fox".replace("fox", "bear");
Alternative:
var regex = new RegExp("/(?!(?:[^<]+>|[^>]+<\/a>))\b(value)\b/", "is");
var regex = new RegExp("/(?!(?:[^<]+>|[^>]+<\/a>))\b(" + value + ")\b/", "is");
var regex = new RegExp("/(?!(?:[^<]+>|[^>]+<\/a>))\b(.*?)\b/", "is");
Keep in mind that if value contains regular expressions characters like (, [ and ? you will need to escape them.

I found this thread useful - so I thought I would add the answer to my own problem.
I wanted to edit a database configuration file (datastax cassandra) from a node application in javascript and for one of the settings in the file I needed to match on a string and then replace the line following it.
This was my solution.
dse_cassandra_yaml='/etc/dse/cassandra/cassandra.yaml'
// a) find the searchString and grab all text on the following line to it
// b) replace all next line text with a newString supplied to function
// note - leaves searchString text untouched
function replaceStringNextLine(file, searchString, newString) {
fs.readFile(file, 'utf-8', function(err, data){
if (err) throw err;
// need to use double escape '\\' when putting regex in strings !
var re = "\\s+(\\-\\s(.*)?)(?:\\s|$)";
var myRegExp = new RegExp(searchString + re, "g");
var match = myRegExp.exec(data);
var replaceThis = match[1];
var writeString = data.replace(replaceThis, newString);
fs.writeFile(file, writeString, 'utf-8', function (err) {
if (err) throw err;
console.log(file + ' updated');
});
});
}
searchString = "data_file_directories:"
newString = "- /mnt/cassandra/data"
replaceStringNextLine(dse_cassandra_yaml, searchString, newString );
After running, it will change the existing data directory setting to the new one:
config file before:
data_file_directories:
- /var/lib/cassandra/data
config file after:
data_file_directories:
- /mnt/cassandra/data

Much easier way: use template literals.
var variable = 'foo'
var expression = `.*${variable}.*`
var re = new RegExp(expression, 'g')
re.test('fdjklsffoodjkslfd') // true
re.test('fdjklsfdjkslfd') // false

Using string variable(s) content as part of a more complex composed regex expression (es6|ts)
This example will replace all urls using my-domain.com to my-other-domain (both are variables).
You can do dynamic regexs by combining string values and other regex expressions within a raw string template. Using String.raw will prevent javascript from escaping any character within your string values.
// Strings with some data
const domainStr = 'my-domain.com'
const newDomain = 'my-other-domain.com'
// Make sure your string is regex friendly
// This will replace dots for '\'.
const regexUrl = /\./gm;
const substr = `\\\.`;
const domain = domainStr.replace(regexUrl, substr);
// domain is a regex friendly string: 'my-domain\.com'
console.log('Regex expresion for domain', domain)
// HERE!!! You can 'assemble a complex regex using string pieces.
const re = new RegExp( String.raw `([\'|\"]https:\/\/)(${domain})(\S+[\'|\"])`, 'gm');
// now I'll use the regex expression groups to replace the domain
const domainSubst = `$1${newDomain}$3`;
// const page contains all the html text
const result = page.replace(re, domainSubst);
note: Don't forget to use regex101.com to create, test and export REGEX code.

var string = "Hi welcome to stack overflow"
var toSearch = "stack"
//case insensitive search
var result = string.search(new RegExp(toSearch, "i")) > 0 ? 'Matched' : 'notMatched'
https://jsfiddle.net/9f0mb6Lz/
Hope this helps

Extending a regex

I'm trying to match javascript files inside /static/js that include ?v=xxxx at the end, 'x' being a character or a number, so it has to match:
http://127.0.0.1:8888/static/js/components/backbone.js?v=a6tsb
But not:
http://127.0.0.1:8888/static/js/views/ribbon.js
http://127.0.0.1:8888/templates/require-config.js
This one matches the hash:
var hashRegex = new RegExp("^.*\\?v=\\w{5}$");
But I'm trying to extend that one to include "/static/js".
I tried:
var hashRegex = new RegExp("^.*\/static\/js\/.*\\?v=\\w{5}$");
But doesn't seems to work.
What am I missing?

In javascript when regex is represented using string you need to double escape(\\) the special character(of regex)
So,your regex would be
var hashRegex = new RegExp("^.*/static/js/.*\\?v=\\w{5}$");
But if you use this syntax for regex
var hashRegex = /regex/;
you have to escape with single \.You would also escape / since it is used as a delimiter
So,your regex in this case would be
var hashRegex = /^.*\/static\/js\/.*\?v=\w{5}$/;

I would try this:
var hashRegex = new RegExp("^.*\/static\/js\/.*\?v\=[a-zA-Z0-9]{5}$");
( I don't know if you have to escape the = )

matching values stored in variables or array with string javascript regex

Im trying to find a patterns in the sentence for regex matching.. in the code below result contains a string and we are checking if the word apple is present in it.
var patt = /apple/gi;
var newResult = patt.test(result);
I found the above code from a used case.. But i was wondering if i have more than one values and i want to check it in the string result, lets say an array with values var arr=["apple", "orange"] var patt=/arr[0]/gi will not work.. what could be the way in that scenario??

To check multiple entries, you can use the OR operator:
var patt = /apple|orange/gi;
var newResult = patt.test(result);
if you have a variable, you can do the below, IF(!) your key is regexp safe of course (that is, it doesn't contains characters which have meaning in regexp syntax):
var key = "apple";
var patt = new RegExp(key, 'gi');
var newResult = patt.test(result);
Although in this case, you might as well use indexOf:
var key = "apple";
var newResult = result.indexOf(key) > -1;

To use a string for your regex expressions, you need to create the regex using the regex constructor.
var pattern = "apple|orange";
var regex = new RegExp(pattern, "g"); // g is for global match
Read more about it here: https://developer.mozilla.org/en-US/docs/JavaScript/Guide/Regular_Expressions