SWI-Prolog -- Manual

Documentation
- Reference manual
  - Overview
    - The SWI-Prolog syntax
      - ISO Syntax Support
        
        Processor Character Set
        
        Nested comments
        
        Character Escape Syntax
        
        Syntax for non-decimal numbers
        
        Using digit groups in large integers
        
        NaN and Infinity floats and their syntax
        
        Force only underscore to introduce a variable
        
        Unicode Prolog source
        
        Singleton variable checking
- Packages

2.16.1.1 Processor Character Set

The processor character set specifies the class of each character used for parsing Prolog source text. Character classification is fixed to http://www.unicode.org/Unicode. See also section 2.19.