Letters (except stressed à), Numbers 0-9 and Hyphen.
http://en.wikipedia.org/wiki/Domain_name
excerpt:
Valid characters that can be used in a domain name are:
a-z
0-9
- but not as a starting or ending character
. as a separator for the textual portions of a domain name
From https://www.rfc-editor.org/rfc/rfc1035
Various objects and parameters in the DNS have size limits. They are
listed below. Some could be easily changed, others are more
fundamental.
labels 63 octets or less
names 255 octets or less
TTL positive values of a signed 32 bit number.
UDP messages 512 octets or less