hosts file format

Hosts file is a plain text file, it's used to combine the IPs to corresponding domains.
According to http://linux.die.net/man/5/hosts, here are the rules of hosts file.

1. Only one IP address in each line.
2. Format in one line,

   \ \ [aliases...]

   These items are separated by one or more Space or Tab.
3. hostname can only contain alphabet, number, hyphen(-) and dot(.). It must begin with alphanumeric and end with alphanumeric.
4. alias is optional, can be many.
5. The content that begins hash(#) ends with line break is regarded as comment.
6. Empty lines are ignored.

So, we can know that:

1. hosts file doesn't support wildcard and regex expression.
   123.123.123.123 *.mydomain.com
   Lines like this are illegal.
2. Comments can be placed at the end of line.
   123.123.123.123 foo.mydomain.com #comments