See in ABNF
Note:this pattern is overly restrictive, the normative definition is type TSimpleIdentifier in OData EDM XML Schema
odataIdentifier = identifierLeadingCharacter *127identifierCharacter
identifierLeadingCharacter = ALPHA / "_" ; plus Unicode characters from the categories L or Nl
identifierCharacter = ALPHA / "_" / DIGIT ; plus Unicode characters from the categories L, Nl, Nd, Mn,
Mc, Pc, or Cf
And in OData V2 MC-CSDL (Release v20110610)
Section 2.2.6 SimpleIdentifier
SimpleIdentifier is a string-based representation. The maximum length of the identifier MUST be less than 480.
The below pattern represents the allowed identifiers in ECMA specification:
Pattern: value="[\p{L}\p{Nl}][\p{L}\p{Nl}\p{Nd}\p{Mn}\p{Mc}\p{Pc}\p{Cf}]{0,}"