Remove the quotations:

pattern = /(\d{1,4}([.\-/])\d{1,2}([.\-/])\d{1,4})/g;

myDate = "01-01-2021".match(pattern);

console.log(myDate);

As with most things in JavaScript, a Regular Expression pattern is an object - specifically, a RegExp object. As explained in MDN:

There are two ways to create a RegExp object: a literal notation and a constructor.

• The literal notation's parameters are enclosed between slashes and do not use quotation marks.
• The constructor function's parameters are not enclosed between slashes but do use quotation marks.

The additional point relevant to your example is that the g flag is added on the end of the literal notation, but as a separate parameter in the constructor function. So either of the following will work:

pattern = /(\d{1,4}([.\-/])\d{1,2}([.\-/])\d{1,4})/g;
pattern = new RegExp('\d{1,4}([.\-/])\d{1,2}([.\-/])\d{1,4})', 'g');

The reason your attempt didn't give an error, but didn't match the string is explained on the MDN page for the match function:

If regexp is a non-RegExp object, it is implicitly converted to a RegExp by using new RegExp(regexp).

So your code was equivalent to this:

pattern = "/(\d{1,4}([.\-/])\d{1,2}([.\-/])\d{1,4})/g";
myDate = str.match(new RegExp(pattern));

When what you wanted was this:

pattern = "(\d{1,4}([.\-/])\d{1,2}([.\-/])\d{1,4})";
myDate = str.match(new RegExp(pattern, "g"));

Ok, after playing about I got it to work by taking the qoutation marks away by doing this.

var pattern=/(\d{1,4}([.\-/])\d{1,2}([.\-/])\d{1,4})/g;
myDate = str.match(pattern);