Language elements:

• keywords:   create,   select,   from,   where,   ...
• identifiers:   Students,   name,   id,   CourseCode,   ...
• operators:   +,   -,   =,   <,   >,   AND,   OR,   NOT,   IN,   ...
• constants:   'abc',   123,   3.1,   '01-jan-1970',   ...

PostgreSQL parser ...

• implemented via lex/yacc   (src/backend/parser)
• maps all identifiers to lower-case   (A-Z → a-z)
• needs to handle user-extendable operator set
• makes extensive use of catalog   (src/backend/catalog)