Dynamic construction of RegEx based on user input

Question

I trying to get user input match it against a string variable, when I passed a literal string to new RegExp(/\bjavascript/), it works, but I tried new RegExp('\b' + this.input) or new RegExp('/\b' + this.input + '/'); it fails.

Here is the code

 var lang = 'javascript'
 let patern =  new RegExp('/\b' + this.input + '/');
 console.log(patern.test('lang'));

where 'input' is declare within my class

Luca Kiebel · Answer 1 · 2018-09-15T14:24:49.543

2

Remove the slash from the beginning and end and escape backslashes:

this.input = "St";

let pattern =  new RegExp('\\b' + this.input, 'i');

console.log(
  "Stack Overflow".match(pattern)
)

edited Sep 15 '18 at 14:24

answered Sep 15 '18 at 13:35

Luca Kiebel

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Thanks Luca that works but i tried adjust it to be case insensitive like: let pattern = new RegExp('\\b' + this.input + '\\i'); but fail again any reason why? – yaxx Sep 15 '18 at 14:23
RegEx takes flags as the second parameter, `new RegExp('\\b' + this.input, 'i');` will give you a case-insensitive regex – Luca Kiebel Sep 15 '18 at 14:25

score 0 · Accepted Answer · answered Sep 15 '18 at 13:48

When input comes from a user, it's a good idea to escape the string using the following:

RegExp.escape = function (s)
{
    return s.replace(/[-\/\\^$*+?.()|[\]{}]/g, '\\$&');
};

You call it like this:

 let patern =  new RegExp('\\b' + RegExp.escape(this.input));

That way it Works even when the input has special characters in it, like '\' or parenthesis-

Dynamic construction of RegEx based on user input

2 Answers2