Use Whitelist RegEx in Javascript to validate a string

Question

I'm trying to prevent an action based on wether a string passes the whiteList Regex in Javascript

const whiteList = /[@A-Za-z0-9.,-]/g // Regex from external source. It will be difficult to modify.
const str= 'ds%d';


console.log(str.replace(whiteList, '').length === 0) // Expected false - WORKS

// How can I make this statement return false ?
console.log(whiteList.test(str)) //Expected false Actual true

Tried using replace command to check if a string passed a validation based on a whitelist. It works but I believe there could be a better way of solving this problem.

T.J. Crowder · Accepted Answer · 2022-11-18 16:22:24Z

1

You get true from test because there is a character in the string that matches the expression. The expression has no anchors, so it's not that it requires all characters in the string to match, just one. To require all characters to match, you'd need a "start of input" assertion (^) at the beginning, an "end of input" assertion ($) at the end, and either a "zero or more" (*) or "one or more" (+) quantifier on the character class (depending on whether an empty string should pass).

If you're getting the expression from elsewhere, you can add those to it after the fact:

const whiteList = /[@A-Za-z0-9.,-]/g // Regex from external source. It will be difficult to modify.
const str= 'ds%d';

console.log(str.replace(whiteList, '').length === 0);

const improvedList = new RegExp("^" + whiteList.source + "+$");
console.log(improvedList.test(str)); // Now shows false

That does make the assumption that the original regex has the problem described. You might check first, but it would be easy to construct regular expressions that seemed like they needed modifying but didn't.

Alternatively, just use the replace check you have, since it works as well. It's not that much more expensive.

answered Nov 18, 2022 at 16:22

T.J. Crowder

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1 Comment

Amarbir Over a year ago

You are right TJ, they needed modifying the Regex. I was able to make the regex change. Now hopefully QA has the capacity to regression :) Thanks, Amar

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