Java RegEx with variable character range

I need to check if all characters of a string are in a specific character range.

I know I can check with the regular expression:

input.matches("[a-z]+");

if a string only contains letters from a to z. But how can I do it if the character range is defined by an given interface? For example, the interface xxx has the attribute

xxx.FIRST_CHAR = 'a';

and

xxx.LAST_CHAR = 'z';

but the attributes can be changed.

Is there any way to use variable characters in regular expressions?