hosts blocks whole domain trees including all descedant domains. The domains in the hosts file must be 2 or 3 segment domains, such as "yahoo.com" and "finance.yahoo.com". If the length of the last 2 segments is less than 7, such as "com.pl", then it is treated as one segment, so domains like "xxx.xxx.com.pl" are also valid in the hosts file.
Each line of the hosts file has the following format:
[rootDomain][space][regex for domain prefix before rootDomain][space][regex for the whole url without "http(s)://"]
The first part [rootDomain] is required and the others are optional. regex is java-grammar regular expression. If the second regex is used, it is recommended to merge the first regex to the second one for performance.