Felix has a rich lexicology for identifiers.
C like identifiers¶
C like identifiers start with a letter, and are optionally followed by a sequence of letters, underscores, single quotes, hyphens, or ASCII digits.
The first character may also be an underscore but that is reserved for the system.
A letter may be any Unicode code point accepted by ISO C++ Standard as a letter, it must be encoded as UTF-8.
Hello X123 a' julie-marx xИΫຜشஇᄗデՖ
Note again please, only ASCII digits are permitted.
A leading slosh followed by a nonempty sequence of ASCII letters is recognised as an identifer. With suitable output machinery, the corresponding LaTeX or AmSTeX symbol should display if there is one.
No space is required after a TeX identifier. Therefore
encodes the symbol alpha followed by the integer 2.