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.


Note again please, only ASCII digits are permitted.

Tex Identifiers

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.


displays as


No space is required after a TeX identifier. Therefore


encodes the symbol alpha followed by the integer 2.