Java Regular expression matching English + digit +-symbol + limit length __ Regular expression

Check the input content with Java code, only allow the combination of English + digit +-symbol, the length can not exceed 20.

Specifically implemented as follows:

    Private Boolean Checkinput (String input) {pattern pattern
        = pattern.compile ("^[a-za-z0-9-]{0,20}$"); 
        Matcher m = pattern.matcher (input);
        if (!m.matches ()) {///////Match is not in, say that the input does not conform to the conditions return
           false; 
        } 
        return true; 
    }

If only the limit is allowed to enter English + digits, the expression should read: ^[a-za-z0-9]+$