正则表达式数值范围，例如1000 - 2000？

I need to check if a string is in the format: xxx[y-z]

Where y and z are two numbers. E.g y can be 1000 and z can be 2000, in which case these will be valid:

xxx1000 xxx1500 xxx1900 xxx2000

Is there a way to accomplish this in regex?

This kind of evalution should not really be in a regex if you can help it. You can use a regex to run a test on the format, but use other functions to test the content:

$in = "xxx1234";
if( preg_match("/^xxx(\d{4})$/",$in,$m) && $m[1] >= 1000 && $m[1] <= 2000) {
    // ok!
}

I don't see the point in using a regex here, especially if y and z are variables. Use the regex to test the format, but then take the right-most n characters (capturing it in a group is the easiest), try to parse it into an integer and test if it's in the range.

If y and z are constant though, sure go ahead:

xxx(2000|1[0-9]{3})$