A variable inside of a pattern? Possible?

[Spring]

If I had the pattern:

$pattern = "/^[a-zA-Z0-9._-]{4,20}$/";

 

And I wanted to replace, 4, 20 with a variable, how would I go about doing so?

[silkfire]

Like this?:

 

$var = '5,19';
$pattern = '/^[a-zA-Z0-9._-]{' . $var . '}$/';