Javascript Regex: How to put a variable inside a regular expression?

user_380562 0
Viewed 240325

So for example:

function(input){
    var testVar = input;
    string = ...
    string.replace(/ReGeX + testVar + ReGeX/, "replacement")
}

But this is of course not working :) Is there any way to do this?

variables javascript regex
9 Answers

You can create regular expressions in JS in one of two ways:

  1. Using regular expression literal - /ab{2}/g
  2. Using the regular expression constructor - new RegExp("ab{2}", "g") .

Regular expression literals are constant, and can not be used with variables. This could be achieved using the constructor. The stracture of the RegEx constructor is 

new RegExp(regularExpressionString, modifiersString)

You can embed variables as part of the regularExpressionString. For example, 

var pattern="cd"
var repeats=3
new RegExp(`${pattern}{${repeats}}`, "g")

This will match any appearance of the pattern cdcdcd.

user_72244300

It's only necessary to prepare the string variable first and then convert it to the RegEx.

for example:

You want to add minLength and MaxLength with the variable to RegEx:

function getRegEx() {
    const minLength = "5"; // for exapmle: min is 5
    const maxLength = "12"; // for exapmle: man is 12

    var regEx = "^.{" + minLength + ","+ maxLength +"}$"; // first we make a String variable of our RegEx
    regEx = new RegExp(regEx, "g"); // now we convert it to RegEx

    return regEx; // In the end, we return the RegEx
}

now if you change value of MaxLength or MinLength, It will change in all RegExs.

Hope to be useful. Also sorry about my English.

user_84390320

accepted answer doesn't work for me and doesn't follow MDN examples

see the 'Description' section in above link

I'd go with the following it's working for me:

let stringThatIsGoingToChange = 'findMe';
let flagsYouWant = 'gi' //simple string with flags
let dynamicRegExp = new RegExp(`${stringThatIsGoingToChange}`, flagsYouWant)

// that makes dynamicRegExp = /findMe/gi
user_99046080
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