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Why is this regex incorrect? [duplicate]

This question already has answers here:
Why do regex constructors need to be double escaped?
(5 answers)
Closed 4 years ago.
I have the following regular expression:
\d{4}(-\d{2}){2}
Which is supposed to match dates that follow the YYYY-MM-DD format, so 1990-01-01 should successfuly match. However this fails when I try it in javascript.
var x = new RegExp('\d{4}(-\d{2}){2}')
x.test('1990-02-01') //why is this false?

Use the regular js regex syntax. Like this:
var x = /\d{4}(-\d{2}){2}/;
console.log(x.test('1990-02-01'));
if you want to keep the new RegExp part, you have to escape the string's backslashes:
var x = new RegExp('\\d{4}(-\\d{2}){2}');
console.log(x.test('1990-02-01'));

using RegExp constructor result into wrong output [duplicate]

This question already has answers here:
Why do regex constructors need to be double escaped?
(5 answers)
Closed 4 years ago.
I have a regex expression which i use for email address validation. It is working fine as literal expression but showing different result when using the RegExp constructor.
var emailpattern=/^(([^<>()\[\]\\.,;:\s#"]+(\.[^<>()\[\]\\.,;:\s#"]+)*)|(".+"))#((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\])|(([a-zA-Z\-0-9]+\.)+[a-zA-Z]{2,}))$/g;
console.log(emailpattern.test('nithin#gmail.com'))//true
var obj = new RegExp('^(([^<>()\[\]\\.,;:\s#"]+(\.[^<>()\[\]\\.,;:\s#"]+)*)|(".+"))#((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\])|(([a-zA-Z\-0-9]+\.)+[a-zA-Z]{2,}))$','g');
console.log(obj.test('nithin#gmail.com'))//false

You need to remove quotes for the pattern which you have specified in the new RegExp() to get rid of extra overhead of escaping the \ character:
var emailpattern=/^(([^<>()\[\]\\.,;:\s#"]+(\.[^<>()\[\]\\.,;:\s#"]+)*)|(".+"))#((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\])|(([a-zA-Z\-0-9]+\.)+[a-zA-Z]{2,}))$/g;
console.log(emailpattern.test('nithin#gmail.com'))//true
var obj = new RegExp(/^(([^<>()\[\]\\.,;:\s#"]+(\.[^<>()\[\]\\.,;:\s#"]+)*)|(".+"))#((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\])|(([a-zA-Z\-0-9]+\.)+[a-zA-Z]{2,}))$/,'g');
console.log(obj.test('nithin#gmail.com'))//false
Escaping the \ character:
var emailpattern=/^(([^<>()\[\]\\.,;:\s#"]+(\.[^<>()\[\]\\.,;:\s#"]+)*)|(".+"))#((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\])|(([a-zA-Z\-0-9]+\.)+[a-zA-Z]{2,}))$/g;
console.log(emailpattern.test('nithin#gmail.com'))//true
var obj = new RegExp('(([^<>()\\[\\]\\\.,;:\s#"]+(\\.[^<>()\\[\\]\\\.,;:\\s#"]+)*)|(".+"))#((\\[[0-9]{1,3}\\.[0-9]{1,3}\\.[0-9]{1,3}\\.[0-9]{1,3}\\])|(([a-zA-Z\\-0-9]+\\.)+[a-zA-Z]{2,}))','g');
console.log(obj.test('nithin#gmail.com'))//false

can't get second number between parenthesis using regex [duplicate]

This question already has answers here:
How do you access the matched groups in a JavaScript regular expression?
(23 answers)
Closed 5 years ago.
now I begin learning regex, and I have set of string in format like "(9/13)", and I need get second number. I try this regex: /\(.*?[^\d]*(\d+?)\)/g, in online regex it works normally.
But here:
var d = "(9/13)";
var v = /\(.*?[^\d]*(\d+?)\)/g;
alert(d.match(v));
it returns "(9/13)" , what am I doing wrong?

const source = "(9/13)";
const re = /\/(\d+)\)/;
console.log('result', re.exec(source).pop())

you can use Regex.exec() to find the number
const source = "(9/13)";
const re = /\(\d+\/(\d+)\)/;
console.log(re.exec(source)[1])

Javascript regex in attribute [duplicate]

This question already has answers here:
Create RegExps on the fly using string variables
(6 answers)
Closed 8 years ago.
I need to use data attribute for regex just as
<div data-regex="REGEX-HERE">
and then get the value by javascript and put in a variable. and then do a test like
var regex = $(this).attr("data-regex");
regex.test(name)
when I tried to use "^[\x20-\x7E]+$" for testing english character is didn't work.
Note when I tried this
var regex = /^[\x20-\x7E]+$/;
It worked.
Thanks in advance

You can do this:
var regex = new RegExp("^[\x20-\x7E]+$",""); // Modifiers on the tend
So finally:
var regex = new RegExp($(this).data("regex"));
regex.test(name)

JS replacing all occurrences of string using variable [duplicate]

This question already has answers here:
How do you use a variable in a regular expression?
(27 answers)
Closed 9 years ago.
I know that str.replace(/x/g, "y")replaces all x's in the string but I want to do this
function name(str,replaceWhat,replaceTo){
str.replace(/replaceWhat/g,replaceTo);
}
How can i use a variable in the first argument?

The RegExp constructor takes a string and creates a regular expression out of it.
function name(str,replaceWhat,replaceTo){
var re = new RegExp(replaceWhat, 'g');
return str.replace(re,replaceTo);
}
If replaceWhat might contain characters that are special in regular expressions, you can do:
function name(str,replaceWhat,replaceTo){
replaceWhat = replaceWhat.replace(/[-\/\\^$*+?.()|[\]{}]/g, '\\$&');
var re = new RegExp(replaceWhat, 'g');
return str.replace(re,replaceTo);
}
See Is there a RegExp.escape function in Javascript?

The third parameter of flags below was removed from browsers a few years ago and this answer is no longer needed -- now replace works global without flags
Replace has an alternate form that takes 3 parameters and accepts a string:
function name(str,replaceWhat,replaceTo){
str.replace(replaceWhat,replaceTo,"g");
}
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/replace