The lexical structure of Swift describes what sequence of characters form valid tokens of the language. These valid tokens form the lowest-level building blocks of the language and are used to describe the rest of the language in subsequent chapters. A token consists of an identifier, keyword, punctuation, literal, or operator.

In most cases, tokens are generated from the characters of a Swift source file by considering the longest possible substring from the input text, within the constraints of the grammar that are specified below. This behavior is referred to as longest match or maximal munch.

Whitespace and Comments

Whitespace has two uses: to separate tokens in the source file and to help determine whether an operator is a prefix or postfix (see Operators), but is otherwise ignored. The following characters are considered whitespace: space (U+0020), line feed (U+000A), carriage return (U+000D), horizontal tab (U+0009), vertical tab (U+000B), form feed (U+000C) and null (U+0000).

Comments are treated as whitespace by the compiler. Single line comments begin with // and continue until a line feed (U+000A) or carriage return (U+000D). Multiline comments begin with /* and end with */. Nesting multiline comments is allowed, but the comment markers must be balanced.

Comments can contain additional formatting and markup, as described in Markup Formatting Reference.

Identifiers

Identifiers begin with an uppercase or lowercase letter A through Z, an underscore (_), a noncombining alphanumeric Unicode character in the Basic Multilingual Plane, or a character outside the Basic Multilingual Plane that isn’t in a Private Use Area. After the first character, digits and combining Unicode characters are also allowed.

To use a reserved word as an identifier, put a backtick (`) before and after it. For example, class is not a valid identifier, but `class` is valid. The backticks are not considered part of the identifier; `x` and x have the same meaning.

Inside a closure with no explicit parameter names, the parameters are implicitly named $0, $1, $2, and so on. These names are valid identifiers within the scope of the closure.

Keywords and Punctuation

The following keywords are reserved and can’t be used as identifiers, unless they’re escaped with backticks, as described above in Identifiers. Keywords other than inout, var, and let can be used as parameter names in a function declaration or function call without being escaped with backticks. When a member has the same name as a keyword, references to that member don’t need to be escaped with backticks, except when there is ambiguity between referring to the member and using the keyword—for example, self, Type, and Protocol have special meaning in an explicit member expression, so they must be escaped with backticks in that context.

• Keywords used in declarations: associatedtype, class, deinit, enum, extension, fileprivate, func, import, init, inout, internal, let, open, operator, private, protocol, public, static, struct, subscript, typealias, and var.

• Keywords used in statements: break, case, continue, default, defer, do, else, fallthrough, for, guard, if, in, repeat, return, switch, where, and while.

• Keywords used in expressions and types: as, Any, catch, false, is, nil, rethrows, super, self, Self, throw, throws, true, and try.

• Keywords used in patterns: _.

• Keywords that begin with a number sign (#): #available, #colorLiteral, #column, #else, #elseif, #endif, #file, #fileLiteral, #function, #if, #imageLiteral, #line, #selector. and #sourceLocation.

• Keywords reserved in particular contexts: associativity, convenience, dynamic, didSet, final, get, infix, indirect, lazy, left, mutating, none, nonmutating, optional, override, postfix, precedence, prefix, Protocol, required, right, set, Type, unowned, weak, and willSet. Outside the context in which they appear in the grammar, they can be used as identifiers.

The following tokens are reserved as punctuation and can’t be used as custom operators: (, ), {, }, [, ], ., ,, :, ;, =, @, #, & (as a prefix operator), ->, `, ?, and ! (as a postfix operator).

Literals

A literal is the source code representation of a value of a type, such as a number or string.

The following are examples of literals:

A literal doesn’t have a type on its own. Instead, a literal is parsed as having infinite precision and Swift’s type inference attempts to infer a type for the literal. For example, in the declaration let x: Int8 = 42, Swift uses the explicit type annotation (: Int8) to infer that the type of the integer literal 42 is Int8. If there isn’t suitable type information available, Swift infers that the literal’s type is one of the default literal types defined in the Swift standard library. The default types are Int for integer literals, Double for floating-point literals, String for string literals, and Bool for Boolean literals. For example, in the declaration let str = "Hello, world", the default inferred type of the string literal "Hello, world" is String.

When specifying the type annotation for a literal value, the annotation’s type must be a type that can be instantiated from that literal value. That is, the type must conform to one of the following Swift standard library protocols: ExpressibleByIntegerLiteral for integer literals, ExpressibleByFloatLiteral for floating-point literals, ExpressibleByStringLiteral for string literals, ExpressibleByBooleanLiteral for Boolean literals, ExpressibleByUnicodeScalarLiteral for string literals that contain only a single Unicode scalar, and ExpressibleByExtendedGraphemeClusterLiteral for string literals that contain only a single extended grapheme cluster. For example, Int8 conforms to the ExpressibleByIntegerLiteral protocol, and therefore it can be used in the type annotation for the integer literal 42 in the declaration let x: Int8 = 42.

Operators

The Swift standard library defines a number of operators for your use, many of which are discussed in Basic Operators and Advanced Operators. The present section describes which characters can be used to define custom operators.

Custom operators can begin with one of the ASCII characters /, =, -, +, !, *, %, <, >, &, |, ^, ?, or ~, or one of the Unicode characters defined in the grammar below (which include characters from the Mathematical Operators, Miscellaneous Symbols, and Dingbats Unicode blocks, among others). After the first character, combining Unicode characters are also allowed.

You can also define custom operators that begin with a dot (.). These operators can contain additional dots. For example, .+. is treated as a single operator. If an operator doesn’t begin with a dot, it can’t contain a dot elsewhere. For example, +.+ is treated as the + operator followed by the .+ operator.

Although you can define custom operators that contain a question mark (?), they can’t consist of a single question mark character only. Additionally, although operators can contain an exclamation mark (!), postfix operators cannot begin with either a question mark or an exclamation mark.

The whitespace around an operator is used to determine whether an operator is used as a prefix operator, a postfix operator, or a binary operator. This behavior is summarized in the following rules:

• If an operator has whitespace around both sides or around neither side, it is treated as a binary operator. As an example, the +++ operator in a+++b and a +++ b is treated as a binary operator.

• If an operator has whitespace on the left side only, it is treated as a prefix unary operator. As an example, the +++ operator in a +++b is treated as a prefix unary operator.

• If an operator has whitespace on the right side only, it is treated as a postfix unary operator. As an example, the +++ operator in a+++ b is treated as a postfix unary operator.

• If an operator has no whitespace on the left but is followed immediately by a dot (.), it is treated as a postfix unary operator. As an example, the +++ operator in a+++.b is treated as a postfix unary operator (a+++ .b rather than a +++ .b).

For the purposes of these rules, the characters (, [, and { before an operator, the characters ), ], and } after an operator, and the characters ,, ;, and : are also considered whitespace.

There is one caveat to the rules above. If the ! or ? predefined operator has no whitespace on the left, it is treated as a postfix operator, regardless of whether it has whitespace on the right. To use the ? as the optional-chaining operator, it must not have whitespace on the left. To use it in the ternary conditional (? :) operator, it must have whitespace around both sides.

In certain constructs, operators with a leading < or > may be split into two or more tokens. The remainder is treated the same way and may be split again. As a result, there is no need to use whitespace to disambiguate between the closing > characters in constructs like Dictionary<String, Array<Int>>. In this example, the closing > characters are not treated as a single token that may then be misinterpreted as a bit shift >> operator.

To learn how to define new, custom operators, see Custom Operators and Operator Declaration. To learn how to overload existing operators, see Operator Methods.