Question

I have a box where the user inputs a regex, and in Javascript I take that value and have another string tested with it like so: (this is an abstraction of my real issue)

var regex = $('input').val();
regex.test('some string');

The only way I know to make sure to cast the regex to a safe Object type, is to use eval().

Is that the best way of casting it?

Answer 1

Use the RegExp constructor to create a pattern.

// The next line would escape special characters. Since you want to support
//    manually created RegExps, the next line is commented:
// regex = regexp.replace(/([[^$.|?*+(){}])/g, '\\$1')

regex = new RegExp(regex);
//                      ^ Optionally, add the flags as a second argument, eg:
//regex=new RegExp(regex, 'i');   //Case-insensitive

UPDATE
You seem to misunderstand the usage of the RegExp constructor. The “slash-notation” is the “primitive” way to create a Regular expression. For comparsion, consider (new is optional):

"123"             ===    new String(123)
false             ===    new Boolean(1)
// Because a RegExp is an object, the strict compare `===` method evaluates to
//  false if the pattern is not the same object.
// Example:   /\d/ == /\d/   evaluates to false
//      To compare a regex pattern, use the `pattern` property
/[a-z]/i.pattern   ===    (new RegExp("[a-z]", "i")).pattern

The RegExp constructor takes two arguments, with the second one being optional:

1. String Pattern (without trailing and ending slashes)

2. String (optional) Flags A combination of:

   • i (ignore case)
   • g (global match)
   • m (multi-line (rarely used)).

Examples (new is optional):

Using constructor                 using slash-notation   # Notice:
RegExp('[0-9]');                /[0-9]/                  # no slashes at RegExp
RegExp('/path/to/file\.html$')  /path\/to\/file\.html$/  # the escaped \ 
RegExp('i\'m', 'i')             /i'm/i                   # \' vs ', 'i' vs /i

Implementing a “RegExp” form field using slash-notation

var regex = $('input').val();  //Example: '/^[0-9]+$/i'
// Using a RegEx to implement a Reg Exp, ironically..
regex = regex.match(/^\/([\S\s]+)\/([gim]{0,3})$/);
regex = regex || [, regex, ""];        // If the previous match is null,
                                     // treat the string as a slash-less RegEx
regex = new RegExp(regex[1], regex[2]);
regex.test('some string');