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
        
        Rational number syntax
        
        NaN and Infinity floats and their syntax
        
        Force only underscore to introduce a variable
        
        Unicode Prolog source
        
        Singleton variable checking
- Packages

2.16.1.5 Using digit groups in large integers

SWI-Prolog supports splitting long integers into digit groups. Digit groups can be separated with the sequence <underscore>, <optional white space>. If the <radix> is 10 or lower, they may also be separated with exactly one space. The following all express the integer 1 million:

1_000_000
1 000 000
1_000_/*more*/000

Integers can be printed using this notation with format/2, using the ~I format specifier. For example:

?- format('~I', [1000000]).
1_000_000

The current syntax has been proposed by Ulrich Neumerkel on the SWI-Prolog mailinglist.