D Programming Language
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# Lexical

The lexical analysis is independent of the syntax parsing and the semantic analysis. The lexical analyzer splits the source text up into tokens. The lexical grammar describes what those tokens are. The grammar is designed to be suitable for high speed scanning, it has a minimum of special case rules, there is only one phase of translation, and to make it easy to write a correct scanner for. The tokens are readily recognizable by those familiar with C and C++.

### Source Text

D source text can be in one of the following formats:

- ASCII
- UTF-8
- UTF-16BE
- UTF-16LE
- UTF-32BE
- UTF-32LE

UTF-8 is a superset of traditional 7-bit ASCII. One of the following UTF BOMs (Byte Order Marks) can be present at the beginning of the source text:

UTF Byte Order Marks
Format | BOM
--- | ---
UTF-8 | EF BB BF
UTF-16BE | FE FF
UTF-16LE | FF FE
UTF-32BE | 00 00 FE FF
UTF-32LE | FF FE 00 00
ASCII | no BOM

If the source file does not start with a BOM, then the first character must be less than or equal to U0000007F.

There are no digraphs or trigraphs in D.

The source text is decoded from its source representation into Unicode Characters. The Characters are further divided into: WhiteSpace, EndOfLine, Comments, SpecialTokenSequences, Tokens, all followed by EndOfFile.

The source text is split into tokens using the maximal munch technique, i.e., the lexical analyzer tries to make the longest token it can. For example >> is a right shift token, not two greater than tokens. An exception to this rule is that a .. embedded inside what looks like two floating point literals, as in 1..2, is interpreted as if the .. was separated by a space from the first integer.

### Character Set

    Character:
        any Unicode character

### End of File

    EndOfFile:
        physical end of the file
        \u0000
        \u001A

The source text is terminated by whichever comes first.

### End of Line

    EndOfLine:
        \u000D
        \u000A
        \u000D \u000A
        \u2028
        \u2029
        EndOfFile

There is no backslash line splicing, nor are there any limits on the length of a line.

### White Space

    WhiteSpace:
        Space
        Space WhiteSpace

    Space:
        \u0020
        \u0009
        \u000B
        \u000C

### Comments

    Comment:
        BlockComment
        LineComment
        NestingBlockComment

    BlockComment:
        /* Characters */

    LineComment:
        // Characters EndOfLine

    NestingBlockComment:
        /+ NestingBlockCommentCharacters +/

    NestingBlockCommentCharacters:
        NestingBlockCommentCharacter
        NestingBlockCommentCharacter NestingBlockCommentCharacters

    NestingBlockCommentCharacter:
        Character
        NestingBlockComment

    Characters:
        Character
        Character Characters

D has three kinds of comments:

1. Block comments can span multiple lines, but do not nest.
2. Line comments terminate at the end of the line.
3. Nesting block comments can span multiple lines and can nest.

The contents of strings and comments are not tokenized. Consequently, comment openings occurring within a string do not begin a comment, and string delimiters within a comment do not affect the recognition of comment closings and nested "/+" comment openings. With the exception of "/+" occurring within a "/+" comment, comment openings within a comment are ignored.

    a = /+ // +/ 1;    // parses as if 'a = 1;'
    a = /+ "+/" +/ 1"; // parses as if 'a = " +/ 1";'
    a = /+ /* +/ */ 3; // parses as if 'a = */ 3;'

Comments cannot be used as token concatenators, for example, abc/**/def is two tokens, abc and def, not one abcdef token.

### Tokens

    Token:
        Identifier
        StringLiteral
        CharacterLiteral
        IntegerLiteral
        FloatLiteral
        Keyword
        /
        /=
        .
        ..
        ...
        &
        &=
        &&
        |
        |=
        ||
        -
        -=
        --
        +
        +=
        ++
        <
        <=
        <<
        <<=
        <>
        <>=
        >
        >=
        >>=
        >>>=
        >>
        >>>
        !
        !=
        !<>
        !<>=
        !<
        !<=
        !>
        !>=
        (
        )
        [
        ]
        {
        }
        ?
        ,
        ;
        :
        $
        =
        ==
        *
        *=
        %
        %=
        ^
        ^=
        ^^
        ^^=
        ~
        ~=
        @
        =>
        #

### Identifiers

    Identifier:
        IdentifierStart
        IdentifierStart IdentifierChars

    IdentifierChars:
        IdentifierChar
        IdentifierChar IdentifierChars

    IdentifierStart:
        _
        Letter
        UniversalAlpha

    IdentifierChar:
        IdentifierStart
        0
        NonZeroDigit

Identifiers start with a letter, _, or universal alpha, and are followed by any number of letters, _, digits, or universal alphas. Universal alphas are as defined in ISO/IEC 9899:1999(E) Appendix D. (This is the C99 Standard.) Identifiers can be arbitrarily long, and are case sensitive. Identifiers starting with __ (two underscores) are reserved.

### String Literals

    StringLiteral:
        WysiwygString
        AlternateWysiwygString
        DoubleQuotedString

        HexString
        DelimitedString
        TokenString

    WysiwygString:
        r" WysiwygCharacters " StringPostfixopt

    AlternateWysiwygString:
        ` WysiwygCharacters ` StringPostfixopt

    WysiwygCharacters:
        WysiwygCharacter
        WysiwygCharacter WysiwygCharacters

    WysiwygCharacter:
        Character
        EndOfLine

    DoubleQuotedString:
        " DoubleQuotedCharacters " StringPostfixopt

    DoubleQuotedCharacters:
        DoubleQuotedCharacter
        DoubleQuotedCharacter DoubleQuotedCharacters

    DoubleQuotedCharacter:
        Character
        EscapeSequence
        EndOfLine

    EscapeSequence:
        \'
        \"
        \?
        \\
        \a
        \b
        \f
        \n
        \r
        \t
        \v
        \ EndOfFile
        \x HexDigit HexDigit
        \ OctalDigit
        \ OctalDigit OctalDigit
        \ OctalDigit OctalDigit OctalDigit
        \u HexDigit HexDigit HexDigit HexDigit
        \U HexDigit HexDigit HexDigit HexDigit HexDigit HexDigit HexDigit HexDigit
        \ NamedCharacterEntity

    HexString:
        x" HexStringChars " StringPostfixopt

    HexStringChars:
        HexStringChar
        HexStringChar HexStringChars

    HexStringChar:
        HexDigit
        WhiteSpace
        EndOfLine

    StringPostfix:
        c
        w
        d

    DelimitedString:
        q" Delimiter WysiwygCharacters MatchingDelimiter "

    TokenString:
        q{ Tokens }

A string literal is either a double quoted string, a wysiwyg quoted string, an escape sequence, a delimited string, a token string, or a hex string.

In all string literal forms, an EndOfLine is regarded as a single \n character.

#### Wysiwyg Strings

Wysiwyg "what you see is what you get" quoted strings are enclosed by r" and ". All characters between the r" and " are part of the string. There are no escape sequences inside r" ":

    r"hello"
    r"c:\root\foo.exe"
    r"ab\n" // string is 4 characters,
            // 'a', 'b', '\', 'n'

An alternate form of wysiwyg strings are enclosed by backquotes, the ` character. The ` character is not available on some keyboards and the font rendering of it is sometimes indistinguishable from the regular ' character. Since, however, the ` is rarely used, it is useful to delineate strings with " in them.

    `hello`
    `c:\root\foo.exe`
    `ab\n`  // string is 4 characters,
            // 'a', 'b', '\', 'n'

#### Double Quoted Strings

Double quoted strings are enclosed by "". Escape sequences can be embedded into them with the typical \ notation.

    "hello"
    "c:\\root\\foo.exe"
    "ab\n"   // string is 3 characters,
             // 'a', 'b', and a linefeed
    "ab
    "        // string is 3 characters,
             // 'a', 'b', and a linefeed

#### Hex Strings

Hex strings allow string literals to be created using hex data. The hex data need not form valid UTF characters.

    x"0A"              // same as "\x0A"
    x"00 FBCD 32FD 0A" // same as
                       // "\x00\xFB\xCD\x32\xFD\x0A"

Whitespace and newlines are ignored, so the hex data can be easily formatted. The number of hex characters must be a multiple of 2.

Adjacent strings are concatenated with the ~ operator, or by simple juxtaposition:

    "hello " ~ "world" ~ "\n" // forms the string
           // 'h','e','l','l','o',' ',
           // 'w','o','r','l','d',linefeed

The following are all equivalent:

    "ab" "c"
    r"ab" r"c"
    r"a" "bc"
    "a" ~ "b" ~ "c"

The optional StringPostfix character gives a specific type to the string, rather than it being inferred from the context. This is useful when the type cannot be unambiguously inferred, such as when overloading based on string type. The types corresponding to the postfix characters are:

String Literal Postfix Characters
Postfix | Type | Aka
--- | --- | ---
c | immutable(char)[] | string
w | immutable(wchar)[] | wstring
d | immutable(dchar)[] | dstring

    "hello"c  // string
    "hello"w  // wstring
    "hello"d  // dstring

The string literals are assembled as UTF-8 char arrays, and the postfix is applied to convert to wchar or dchar as necessary as a final step.

String literals are read only. Writes to string literals cannot always be detected, but cause undefined behavior.

#### Delimited Strings

Delimited strings use various forms of delimiters. The delimiter, whether a character or identifer, must immediately follow the " without any intervening whitespace. The terminating delimiter must immediately precede the closing " without any intervening whitespace. A nesting delimiter nests, and is one of the following characters:

Nesting Delimiters
Delimiter | Matching Delimiter
--- | ---
[ | ]
( | )
< | >
{ | }

    q"(foo(xxx))"   // "foo(xxx)"
    q"[foo{]"       // "foo{"

If the delimiter is an identifier, the identifier must be immediately followed by a newline, and the matching delimiter is the same identifier starting at the beginning of the line:

    writefln(q"EOS
    This
    is a multi-line
    heredoc string
    EOS"
    );

The newline following the opening identifier is not part of the string, but the last newline before the closing identifier is part of the string. The closing identifier must be placed on its own line at the leftmost column.

Otherwise, the matching delimiter is the same as the delimiter character:

    q"/foo]/"          // "foo]"
    // q"/abc/def/"    // error

#### Token Strings

Token strings open with the characters q{ and close with the token }. In between must be valid D tokens. The { and } tokens nest. The string is formed of all the characters between the opening and closing of the token string, including comments.

    q{foo}              // "foo"
    q{/*}*/ }           // "/*}*/ "
    q{ foo(q{hello}); } // " foo(q{hello}); "
    q{ __TIME__ }       // " __TIME__ "
        // i.e. it is not replaced with the time
    // q{ __EOF__ }     // error
        // __EOF__ is not a token, it's end of file

### Character Literals

    CharacterLiteral:
        ' SingleQuotedCharacter '

    SingleQuotedCharacter:
        Character
        EscapeSequence

Character literals are a single character or escape sequence enclosed by single quotes, ' '.

### Integer Literals

    IntegerLiteral:
        Integer
        Integer IntegerSuffix

    Integer:
        DecimalInteger
        BinaryInteger
        HexadecimalInteger

    IntegerSuffix:
        L
        u
        U
        Lu
        LU
        uL
        UL

    DecimalInteger:
        0
        NonZeroDigit
        NonZeroDigit DecimalDigitsUS

    BinaryInteger:
        BinPrefix BinaryDigits

    BinPrefix:
        0b
        0B

    HexadecimalInteger:
        HexPrefix HexDigitsNoSingleUS

    NonZeroDigit:
        1
        2
        3
        4
        5
        6
        7
        8
        9

    DecimalDigits:
        DecimalDigit
        DecimalDigit DecimalDigits

    DecimalDigitsUS:
        DecimalDigitUS
        DecimalDigitUS DecimalDigitsUS

    DecimalDigitsNoSingleUS:
        DecimalDigit
        DecimalDigit DecimalDigitsUS
        DecimalDigitsUS DecimalDigit

    DecimalDigitsNoStartingUS:
        DecimalDigit
        DecimalDigit DecimalDigitsUS

    DecimalDigit:
        0
        NonZeroDigit

    DecimalDigitUS:
        DecimalDigit
        _

    BinaryDigitsUS:
        BinaryDigitUS
        BinaryDigitUS BinaryDigitsUS

    BinaryDigit:
        0
        1

    BinaryDigitUS:
        BinaryDigit
        _

    OctalDigits:
        OctalDigit
        OctalDigit OctalDigits

    OctalDigitsUS:
        OctalDigitUS
        OctalDigitUS OctalDigitsUS

    OctalDigit:
        0
        1
        2
        3
        4
        5
        6
        7

    OctalDigitUS:
        OctalDigit
        _

    HexDigits:
        HexDigit
        HexDigit HexDigits

    HexDigitsUS:
        HexDigitUS
        HexDigitUS HexDigitsUS

    HexDigitsNoSingleUS:
        HexDigit
        HexDigit HexDigitsUS
        HexDigitsUS HexDigit

    HexDigit:
        DecimalDigit
        HexLetter

    HexLetter:
        a
        b
        c
        d
        e
        f
        A
        B
        C
        D
        E
        F
        _

Integers can be specified in decimal, binary, octal, or hexadecimal.

Decimal integers are a sequence of decimal digits.

Binary integers are a sequence of binary digits preceded by a ‘0b’.

C-style octal integer notation was deemed too easy to mix up with decimal notation. The above is only fully supported in string literals. D still supports octal integer literals interpreted at compile time through the std.conv.octal template, as in octal!167.

Hexadecimal integers are a sequence of hexadecimal digits preceded by a ‘0x’.

Integers can have embedded ‘_’ characters, which are ignored. The embedded ‘_’ are useful for formatting long literals, such as using them as a thousands separator:

    123_456       // 123456
    1_2_3_4_5_6_  // 123456

Integers can be immediately followed by one ‘L’ or one of ‘u’ or ‘U’ or both. Note that there is no ‘l’ suffix.

The type of the integer is resolved as follows:

Decimal Literal Types
Literal | Type
--- | ---
Usual decimal notation
0 .. 2_147_483_647 | int
2_147_483_648 .. 9_223_372_036_854_775_807 | long
Explicit suffixes
0L .. 9_223_372_036_854_775_807L | long
0U .. 4_294_967_296U | uint
4_294_967_296U .. 18_446_744_073_709_551_615U | ulong
0UL .. 18_446_744_073_709_551_615UL | ulong
Hexadecimal notation
0x0 .. 0x7FFF_FFFF | int
0x8000_0000 .. 0xFFFF_FFFF | uint
0x1_0000_0000 .. 0x7FFF_FFFF_FFFF_FFFF | long
0x8000_0000_0000_0000 .. 0xFFFF_FFFF_FFFF_FFFF | ulong
Hexadecimal notation with explicit suffixes
0x0L .. 0x7FFF_FFFF_FFFF_FFFFL | long
0x8000_0000_0000_0000L .. 0xFFFF_FFFF_FFFF_FFFFL | ulong
0x0U .. 0xFFFF_FFFFU | uint
0x1_0000_0000U .. 0xFFFF_FFFF_FFFF_FFFFU | ulong
0x0UL .. 0xFFFF_FFFF_FFFF_FFFFUL | ulong

### Floating Point Literals

    FloatLiteral:
        Float
        Float Suffix
        Integer ImaginarySuffix
        Integer FloatSuffix ImaginarySuffix
        Integer RealSuffix ImaginarySuffix

    Float:
        DecimalFloat
        HexFloat

    DecimalFloat:
        LeadingDecimal .
        LeadingDecimal . DecimalDigits
        DecimalDigits . DecimalDigitsNoSingleUS DecimalExponent
        . DecimalInteger
        . DecimalInteger DecimalExponent
        LeadingDecimal DecimalExponent

    DecimalExponent
        DecimalExponentStart DecimalDigitsNoSingleUS

    DecimalExponentStart
        e
        E
        e+
        E+
        e-
        E-

    HexFloat:
        HexPrefix HexDigitsNoSingleUS . HexDigitsNoSingleUS HexExponent
        HexPrefix . HexDigitsNoSingleUS HexExponent
        HexPrefix HexDigitsNoSingleUS HexExponent

    HexPrefix:
        0x
        0X

    HexExponent:
        HexExponentStart DecimalDigitsNoSingleUS

    HexExponentStart:
        p
        P
        p+
        P+
        p-
        P-

    Suffix:
        FloatSuffix
        RealSuffix
        ImaginarySuffix
        FloatSuffix ImaginarySuffix
        RealSuffix ImaginarySuffix

    FloatSuffix:
        f
        F

    RealSuffix:
        L

    ImaginarySuffix:
        i

    LeadingDecimal:
        DecimalInteger
        0 DecimalDigitsNoSingleUS

Floats can be in decimal or hexadecimal format.

Hexadecimal floats are preceded with a 0x and the exponent is a p or P followed by a decimal number serving as the exponent of 2.

Floating literals can have embedded ‘_’ characters, which are ignored. The embedded ‘_’ are useful for formatting long literals to make them more readable, such as using them as a thousands separator:

    123_456.567_8         // 123456.5678
    1_2_3_4_5_6_._5_6_7_8 // 123456.5678
    1_2_3_4_5_6_._5e-6_   // 123456.5e-6

Floating literals with no suffix are of type double. Floats can be followed by one f, F, or L suffix. The f or F suffix means it is a float, and L means it is a real.

If a floating literal is followed by i, then it is an ireal (imaginary) type.

Examples:

    0x1.FFFFFFFFFFFFFp1023 // double.max
    0x1p-52                // double.epsilon
    1.175494351e-38F       // float.min
    6.3i                   // idouble 6.3
    6.3fi                  // ifloat 6.3
    6.3Li                  // ireal 6.3

It is an error if the literal exceeds the range of the type. It is not an error if the literal is rounded to fit into the significant digits of the type.

Complex literals are not tokens, but are assembled from real and imaginary expressions during semantic analysis:

    4.5 + 6.2i  // complex number (phased out)

### Keywords

Keywords are reserved identifiers.

    Keyword:
        abstract
        alias
        align
        asm
        assert
        auto

        body
        bool
        break
        byte

        case
        cast
        catch
        cdouble
        cent
        cfloat
        char
        class
        const
        continue
        creal

        dchar
        debug
        default
        delegate
        delete
        deprecated
        do
        double

        else
        enum
        export
        extern

        false
        final
        finally
        float
        for
        foreach
        foreach_reverse
        function

        goto

        idouble
        if
        ifloat
        immutable
        import
        in
        inout
        int
        interface
        invariant
        ireal
        is

        lazy
        long

        macro
        mixin
        module

        new
        nothrow
        null

        out
        override

        package
        pragma
        private
        protected
        public
        pure
        real
        ref
        return

        scope
        shared
        short
        static
        struct
        super
        switch
        synchronized

        template
        this
        throw
        true
        try
        typedef
        typeid
        typeof

        ubyte
        ucent
        uint
        ulong
        union
        unittest
        ushort

        version
        void
        volatile

        wchar
        while
        with

        __FILE__
        __LINE__
        __gshared
        __traits
        __vector
        __parameters

### Special Tokens

These tokens are replaced with other tokens according to the following table:

Special Tokens
Special Token | Replaced with
--- | ---
__DATE__ | string literal of the date of compilation "mmm dd yyyy"
__EOF__ | sets the scanner to the end of the file
__TIME__ | string literal of the time of compilation "hh:mm:ss"
__TIMESTAMP__ | string literal of the date and time of compilation "www mmm dd hh:mm:ss yyyy"
__VENDOR__ | Compiler vendor string, such as "Digital Mars D"
__VERSION__ | Compiler version as an integer, such as 2001

### Special Token Sequences

    SpecialTokenSequence:
        # line IntegerLiteral EndOfLine
        # line IntegerLiteral Filespec EndOfLine

    Filespec:
        " Characters "

Special token sequences are processed by the lexical analyzer, may appear between any other tokens, and do not affect the syntax parsing.

There is currently only one special token sequence, #line.

This sets the source line number to IntegerLiteral, and optionally the source file name to Filespec, beginning with the next line of source text. The source file and line number is used for printing error messages and for mapping generated code back to the source for the symbolic debugging output.

For example:

    int #line 6 "foo\bar"
    x;  // this is now line 6 of file foo\bar

Note that the backslash character is not treated specially inside Filespec strings.

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