What are the allowed characters in a subdomain?

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

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