Why is there result divergence depending on how the regular expression is executed in Javascript?

Question

What is the reason for the result divergence found when executing the same regular expression in different ways in Javascript?

Regular expression:

^([a-z][a-z0-9]{0,30}\.)?((?!\d+\.)[a-z0-9](?:[a-z0-9-]{0,24}[a-z0-9])?)(\.[a-z]{2,4}(?:\.[a-z]{2})?)$

Method 1 (works properly):

function valida_dominio(value){
    return /^([a-z][a-z0-9]{0,30}\.)?((?!\d+\.)[a-z0-9](?:[a-z0-9-]{0,24}[a-z0-9])?)(\.[a-z]{2,4}(?:\.[a-z]{2})?)$/.test(value);
}

Method 2 (not functioning properly):

function valida_dominio(value){
      let str_pattern = '^([a-z][a-z0-9]{0,30}\.)?((?!\d+\.)[a-z0-9](?:[a-z0-9-]{0,24}[a-z0-9])?)(\.[a-z]{2,4}(?:\.[a-z]{2})?)$';
      let pattern = new RegExp(str_pattern, "i");
      return pattern.test(value);
}

The goal of method 2 is to pass regular expression as a string. But it does not work properly. For example:

• approves the domain "cëa.br" which is invalid;
• approves the domain "26characters-asdfasdfa.com.br" which is valid.

How to make method 2 work properly? That is, how to pass the regular expression via string without having divergence in the result?

OBS: No Regexp and in method 1 the regular expression works correctly.

Answer 1

When you use the backslash \ within a string, you are escaping the next character.

I mean, when you write "\." in literal string, your interpreter will understand as if you were trying to escape ., when in fact you wanted to express the literal value \..

So to write an equivalent regular expression like string, you would need to escape the backslash itself, i.e.:

"^([a-z][a-z0-9]{0,30}\\.)?((?!\\d+\\.)[a-z0-9](?:[a-z0-9-]{0,24}[a-z0-9])?)(\\.[a-z]{2,4}(?:\\.[a-z]{2})?)$"