In Syntax§
See primary documentation in context for Number literals
Number literals are generally specified in base ten (which can be specified literally, if needed, via the prefix 0d), unless a prefix like 0x (hexadecimal, base 16), 0o (octal, base 8) or 0b (binary, base 2) or an explicit base in adverbial notation like :16<A0> specifies it otherwise. Unlike other programming languages, leading zeros do not indicate base 8; instead a compile-time warning is issued.
In all literal formats, you can use underscores to group digits, although they don't carry any semantic information; the following literals all evaluate to the same number:
10000001_000_00010_00000100_00_00