ややこしいな https://github.com/apple/swift-syntax/blob/main/Sources/SwiftParser/Lexer/Cursor.swift#L1267C3-L1276C82

/// lexNumber: /// integer_literal ::= [0-9][0-9_]* /// integer_literal ::= 0x[0-9a-fA-F][0-9a-fA-F_]* /// integer_literal ::= 0o[0-7][0-7_]* /// integer_literal ::= 0b[01][01_]* /// floating_literal ::= [0-9][0-9]_*\.[0-9][0-9_]* /// floating_literal ::= [0-9][0-9]*\.[0-9][0-9_]*[eE][+-]?[0-9][0-9_]* /// floating_literal ::= [0-9][0-9_]*[eE][+-]?[0-9][0-9_]* /// floating_literal ::= 0x[0-9A-Fa-f][0-9A-Fa-f_]* /// (\.[0-9A-Fa-f][0-9A-Fa-f_]*)?[pP][+-]?[0-9][0-9_]*

(edited)