How to use a variable inside a RegEx pattern?

Question

Usage of this code is to show only the last four digits of an input.

Here I want to replace this "(/.(?=.{4})/g, '*')" '4' with variable 'mask' . Something like that.

x.value = x.value.replace(/.(?=.{+mask+})/g, '*');

Any suggestion plz

function myFunction(mask) {
    var x = document.getElementById("cc");
    x.value = x.value.replace(/.(?=.{4})/g, '*');
}

<input type="text" id="cc" onkeyup="myFunction(4)" style="text-align:right">

user2864740 · Accepted Answer · 2017-08-02T04:47:17.187

39

Use new RegExp(string) to build a regular expression dynamically. The literal /../ form cannot be used with dynamic content.

Make sure to have a valid pattern after building the string.

var len = 99;
var re = new RegExp(".(?=.{" + len + "})", "g");
var output = input.replace(re, "*")

Also see (and vote for dupe of):

edited Aug 02 '17 at 04:47

answered Aug 02 '17 at 04:41

user2864740

5

Thanks a lot, dude. it works now. "The literal /../ form cannot be used with dynamic content." this line is was the most helpful. – xyzabc Aug 02 '17 at 04:56
How does one convert from a literal form to one that new RegExp can use? Just removing the / doesn't seem to work – Crhistian Ramirez Aug 15 '19 at 17:20
@CrhistianRamirez - It depends on what is in the “/ literal /”. The resulting string of the regular expression must be valid. – user2864740 Aug 15 '19 at 19:13
1

Turns out I wasnt escaping some characters I should have. Specifically the backslash – Crhistian Ramirez Aug 15 '19 at 19:16

1 Answers1