Lexical

Contents
  1. Source Text
  2. Character Set
  3. End of File
  4. End of Line
  5. White Space
  6. Comments
  7. Tokens
  8. Identifiers
  9. String Literals
    1. Wysiwyg Strings
    2. Double Quoted Strings
    3. Hex Strings
    4. Delimited Strings
    5. Token Strings
    6. String Postfix
  10. Escape Sequences
  11. Character Literals
  12. Integer Literals
  13. Floating Point Literals
  14. Keywords
  15. Special Tokens
  16. Special Token Sequences

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 the syntax of these tokens. The grammar is designed to be suitable for high speed scanning and to make it easy to write a correct scanner. It has a minimum of special case rules and there is only one phase of translation.

Source Text

Source text can be in any one of the following encodings:

  • ASCII (strictly, 7-bit ASCII)
  • UTF-8
  • UTF-16BE
  • UTF-16LE
  • UTF-32BE
  • UTF-32LE

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 U+0000007F.

The source text is decoded from its source representation into Unicode Characters. The Characters are further divided into: WhiteSpace, EndOfLine, Comments, SpecialTokenSequences, and Tokens, with the source terminated by an EndOfFile.

The source text is split into tokens using the maximal munch algorithm, i.e., the lexical analyzer makes the longest possible token. For example >> is a right shift token, not two greater than tokens. There are two exceptions to this rule:

  • 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.
  • A 1.a is interpreted as the three tokens 1, ., and a, whereas 1. a is interpreted as the two tokens 1. and a.

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

White Space

WhiteSpace:
    Space
    Space WhiteSpace

Space:
    \u0020
    \u0009
    \u000B
    \u000C

Comments

Comment:
    BlockComment
    LineComment
    NestingBlockComment

BlockComment:
    /* Charactersopt */

LineComment:
    // Charactersopt EndOfLine

NestingBlockComment:
    /+ NestingBlockCommentCharactersopt +/

NestingBlockCommentCharacters:
    NestingBlockCommentCharacter
    NestingBlockCommentCharacter NestingBlockCommentCharacters

NestingBlockCommentCharacter:
    Character
    NestingBlockComment

Characters:
    Character
    Character Characters

There are 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 of the C99 Standard. Identifiers can be arbitrarily long, and are case sensitive.

Implementation Defined: Identifiers starting with __ (two underscores) are reserved.

String Literals

StringLiteral:
    WysiwygString
    AlternateWysiwygString
    DoubleQuotedString

    HexString
    DelimitedString
    TokenString

WysiwygString:
    r" WysiwygCharactersopt " StringPostfixopt

AlternateWysiwygString:
    ` WysiwygCharactersopt ` StringPostfixopt

WysiwygCharacters:
    WysiwygCharacter
    WysiwygCharacter WysiwygCharacters

WysiwygCharacter:
    Character
    EndOfLine

DoubleQuotedString:
    " DoubleQuotedCharactersopt " StringPostfixopt

DoubleQuotedCharacters:
    DoubleQuotedCharacter
    DoubleQuotedCharacter DoubleQuotedCharacters

DoubleQuotedCharacter:
    Character
    EscapeSequence
    EndOfLine

EscapeSequence:
    \'
    \"
    \?
    \\
    \0
    \a
    \b
    \f
    \n
    \r
    \t
    \v
    \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" HexStringCharsopt " StringPostfixopt

HexStringChars:
    HexStringChar
    HexStringChar HexStringChars

HexStringChar:
    HexDigit
    WhiteSpace
    EndOfLine

StringPostfix:
    c
    w
    d

DelimitedString:
    q" Delimiter WysiwygCharactersopt MatchingDelimiter "

TokenString:
    q{ Tokensopt }

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

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

String literals are read only.

Undefined Behavior: Writes to string literals cannot always be detected, but cause undefined behavior.

Wysiwyg Strings

Wysiwyg ("what you see is what you get") quoted strings can be defined using either of two syntaxes.

In the first form, they are enclosed between r" and ". All characters between the r" and " are part of the string. There are no escape sequences inside wysiwyg strings. 

r"I am Oz"
r"c:\games\Sudoku.exe"
r"ab\n" // string is 4 characters,
        // 'a', 'b', '\', 'n'

Alternatively, wysiwyg strings can be enclosed by backquotes, using the ` character. 

`the Great and Powerful.`
`c:\games\Empire.exe`
`The "lazy" dog`
`a"b\n`  // string is 5 characters,
         // 'a', '"', 'b', '\', 'n'

Double Quoted Strings

Double quoted strings are enclosed by "". EscapeSequences can be embedded in them.

"Who are you?"
"c:\\games\\Doom.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.

Note: Hex Strings are deprecated. Please use std.conv.hexString instead.

Delimited Strings

Delimited strings use various forms of delimiters. The delimiter, whether a character or identifier, 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 must be the same identifier starting at the beginning of the line: 

writeln(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{this is the voice of} // "this is the voice of"
q{/*}*/ }               // "/*}*/ "
q{ world(q{control}); } // " world(q{control}); "
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

String Postfix

The optional StringPostfix character gives a specific type to the string, rather than it being inferred from the context. The types corresponding to the postfix characters are:

String Literal Postfix Characters
Postfix Type Alias
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.

Escape Sequences

The escape sequences listed in EscapeSequence are:

Escape Sequences
Sequence Meaning
' Literal single-quote: '
" Literal double-quote: "
? Literal question mark: ?
\ Literal backslash:
\0 Binary zero (NUL, U+0000).
\a BEL (alarm) character (U+0007).
\b Backspace (U+0008).
\f Form feed (FF) (U+000C).
\n End-of-line (U+000A).
\r Carriage return (U+000D).
\t Horizontal tab (U+0009).
\v Vertical tab (U+000B).
\xnn Byte value in hexadecimal, where nn is specified as two hexadecimal digits.
For example: \xFF represents the character with the value 255.
n
nn
nnn 		Byte value in octal.
For example: \101 represents the character with the value 65 ('A'). Analogous to hexadecimal characters, the largest byte value is \377 (= \xFF in hexadecimal or 255 in decimal)
\unnnn Unicode character U+nnnn, where nnnn are four hexadecimal digits.
For example, \u03B3 represents the Unicode character γ (U+03B3 - GREEK SMALL LETTER GAMMA).
\Unnnnnnnn Unicode character U+nnnnnnnn, where nnnnnnnn are 8 hexadecimal digits.
For example, \U0001F603 represents the Unicode character U+1F603 (SMILING FACE WITH OPEN MOUTH).
name Named character entity from the HTML5 specification. These names begin with & and end with ; e.g. &$D(euro;). See NamedCharacterEntity.

Character Literals

CharacterLiteral:
    ' SingleQuotedCharacter '

SingleQuotedCharacter:
    Character
    EscapeSequence

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

'h'   // the letter h
'\n'  // newline
'\\'  // the backslash character

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 BinaryDigitsNoSingleUS

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
    _

BinaryDigitsNoSingleUS:
    BinaryDigit
    BinaryDigit BinaryDigitsUS
    BinaryDigitsUS BinaryDigit
    BinaryDigitsUS BinaryDigit BinaryDigitsUS

BinaryDigitsUS:
    BinaryDigitUS
    BinaryDigitUS BinaryDigitsUS

BinaryDigit:
    0
    1

BinaryDigitUS:
    BinaryDigit
    _

OctalDigit:
    0
    1
    2
    3
    4
    5
    6
    7

HexDigits:
    HexDigit
    HexDigit HexDigits

HexDigitsUS:
    HexDigitUS
    HexDigitUS HexDigitsUS

HexDigitsNoSingleUS:
    HexDigit
    HexDigit HexDigitsUS
    HexDigitsUS HexDigit

HexDigitsNoStartingUS:
    HexDigit
    HexDigit HexDigitsUS

HexDigit:
    DecimalDigit
    HexLetter

HexDigitUS:
    HexDigit
    _

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

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

Decimal integers are a sequence of decimal digits.

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

C-style octal integer notation was deemed too easy to mix up with decimal notation; it 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’ or ‘0X’.

Integers can have embedded ‘_’ characters to improve readability, and which are ignored. 

20_000        // leagues under the sea
867_5309      // number on the wall
1_522_000     // thrust of F1 engine (lbf sea level)

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
9_223_372_036_854_775_808 .. 18_446_744_073_709_551_615 ulong
Explicit suffixes
0L .. 9_223_372_036_854_775_807L long
0U .. 4_294_967_295U 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

An integer literal may not exceed these values.

Best Practices: Octal integer notation is not supported for integer literals. However, octal integer literals can be interpreted at compile time through the std.conv.octal template, as in octal!167.

Floating Point Literals

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

Float:
    DecimalFloat
    HexFloat

DecimalFloat:
    LeadingDecimal .
    LeadingDecimal . DecimalDigits
    DecimalDigits . DecimalDigitsNoStartingUS DecimalExponent
    . DecimalInteger
    . DecimalInteger DecimalExponent
    LeadingDecimal DecimalExponent

DecimalExponent
    DecimalExponentStart DecimalDigitsNoSingleUS

DecimalExponentStart
    e
    E
    e+
    E+
    e-
    E-

HexFloat:
    HexPrefix HexDigitsNoSingleUS . HexDigitsNoStartingUS HexExponent
    HexPrefix . HexDigitsNoStartingUS 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 by a 0x or 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 to improve readability, and which are ignored. 

2.645_751
6.022140857E+23
6_022.140857E+20
6_022_.140_857E+20_

Floating literals with no suffix are of type double. Floating literals followed by f or F are of type float, and those followed by L are of type 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

The literal may not exceed the range of the type. The literal is rounded to fit into the significant digits of the type.

If a floating literal has a . and a type suffix, at least one digit must be in-between:

1f;  // OK, float
1.f; // error
1.;  // OK, double

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)
    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 (reserved)
    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
    typeid
    typeof

    ubyte
    ucent
    uint
    ulong
    union
    unittest
    ushort

    version
    void

    wchar
    while
    with

    __FILE__
    __FILE_FULL_PATH__
    __MODULE__
    __LINE__
    __FUNCTION__
    __PRETTY_FUNCTION__

    __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__ tells the scanner to ignore everything after this token
__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
__VERSION__ Compiler version as an integer
Implementation Defined: The replacement string literal for __VENDOR__ and the replacement integer value for __VERSION__.

Special Token Sequences

SpecialTokenSequence:
    # line IntegerLiteral EndOfLine
    # line IntegerLiteral Filespec EndOfLine

Filespec:
    " Charactersopt "

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 current source line number to IntegerLiteral, and optionally the current source file name to Filespec, beginning with the next line of source text.

The backslash character is not treated specially inside Filespec strings.

For example: 

int #line 6 "pkg/mod.d"
x;  // this is now line 6 of file pkg/mod.d
Implementation Defined: The source file and line number is typically used for printing error messages and for mapping generated code back to the source for the symbolic debugging output.

© 1999–2021 The D Language Foundation
Licensed under the Boost License 1.0.
https://dlang.org/spec/lex.html