This should work. I used

/^(+?d{1,3}|d{1,4})$/

See results

Edit:

The //gm flags are global and multiline, respectively. You need those if you have a string that can have multiple places to match a country code, or there are multiple lines in your string. If your string is going to be more than just a possible country code, you'd need to get rid of the ^ and $ at the beginning and end of the regex. To use the regex, you'd want something like this:

var regex = /^(+?d{1,3}|d{1,4})$/gm

var str = +123

var match = str.match(regex);

//match is an array, with one result in this case. So match[0] == +123