The exact wording in documentation  (emphasis Splunk's):
If the expression references a field name that contains non-alphanumeric characters, other than the underscore ( _ ) character, the field name needs to be surrounded by single quotation marks. For example, if the field name is server-1 you specify the field name like this new=count+'server-1'.
I.e. Any character other then 0-9, A-Z, a-z, and _.