The names of standard attributes and managed attributes in the data store must be in an NCName format.
The NCName format is defined in the W3C document Namespaces in XML 1.0 (Second Edition), located at this URL: http://www.w3.org/TR/REC-xml-names/#NT-NCName
As defined in the W3C document, an NCName must start with either a letter or an underscore (but keep in mind that the W3C definition of Letter includes many non-Latin characters). If the name has more than one character, it must be followed by any combination of letters, digits, periods, dashes, underscores, combining characters, and extenders. (See the W3C document for definitions of combining characters and extenders.) The NCName cannot have colons or white space.
After creating the Endeca attribute, you can use the mdex-property_DisplayName property on the PDR to specify a display name. The display name, which can use a non-NCName format, is intended to serve as an easy-to-understand name for the data store's attribute when it is displayed in the application's front end (such as in Studio's Results Table component).