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

Source Text

Source text can be in one of the following formats:

One of the following UTF BOMs (Byte Order Marks) can be present at the beginning of the source text:

UTF Byte Order Marks
UTF-32BE00 00 FE FF
UTF-32LEFF FE 00 00

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, Tokens, all followed by EndOfFile.

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

Character Set

    any Unicode character

End of File

    physical end of the file

The source text is terminated by whichever comes first.

End of Line

    \u000D \u000A

White Space

    Space WhiteSpace
Space: \u0020 \u0009 \u000B \u000C


BlockComment: /* Characters */
LineComment: // Characters EndOfLine
NestingBlockComment: /+ NestingBlockCommentCharacters +/
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.




    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

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: \' \" \? \\ \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" 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, 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 are enclosed by 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"ab\n" // string is 4 characters,
        // 'a', 'b', '\', 'n'

An alternate form of wysiwyg strings are enclosed by backquotes, the ` character.

`the Great and Powerful.`
`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 into them.

"Who are you?"
"ab\n"   // string is 3 characters,
         // 'a', 'b', and a linefeed
"        // 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 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
DelimiterMatching 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:

is a multi-line
heredoc string

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
"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
\'Literal single-quote: '
\"Literal double-quote: "
\?Literal question mark: ?
\\Literal backslash: \
\0Binary zero (NUL, U+0000).
\aBEL (alarm) character (U+0007).
\bBackspace (U+0008).
\fForm feed (FF) (U+000C).
\nEnd-of-line (U+000A).
\rCarriage return (U+000D).
\tHorizontal tab (U+0009).
\vVertical tab (U+000B).
\xnnByte value in hexadecimal, where nn is specified as two hexadecimal digits.
For example: \xFF represents the character with the value 255.
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)
\unnnnUnicode character U+nnnn, where nnnn are four hexadecimal digits.
For example, \u03B3 represents the Unicode character γ (U+03B3 - GREEK SMALL LETTER GAMMA).
\UnnnnnnnnUnicode character U+nnnnnnnn, where nnnnnnnn are 8 hexadecimal digits.
For example, \U0001F603 represents the Unicode character U+1F603 (SMILING FACE WITH OPEN MOUTH).
\nameNamed character entity from the HTML5 specification. See NamedCharacterEntity.

Character Literals

    ' 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

    Integer IntegerSuffix
Integer: DecimalInteger BinaryInteger HexadecimalInteger
IntegerSuffix: L u U Lu LU uL UL
DecimalInteger: 0 NonZeroDigit NonZeroDigit DecimalDigitsUS
BinaryInteger: BinPrefix BinaryDigitsUS
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 _
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, 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
Usual decimal notation
0 .. 2_147_483_647int
2_147_483_648 .. 9_223_372_036_854_775_807long
Explicit suffixes
0L .. 9_223_372_036_854_775_807Llong
0U .. 4_294_967_295Uuint
4_294_967_296U .. 18_446_744_073_709_551_615Uulong
0UL .. 18_446_744_073_709_551_615ULulong
Hexadecimal notation
0x0 .. 0x7FFF_FFFFint
0x8000_0000 .. 0xFFFF_FFFFuint
0x1_0000_0000 .. 0x7FFF_FFFF_FFFF_FFFFlong
0x8000_0000_0000_0000 .. 0xFFFF_FFFF_FFFF_FFFFulong
Hexadecimal notation with explicit suffixes
0x0L .. 0x7FFF_FFFF_FFFF_FFFFLlong
0x8000_0000_0000_0000L .. 0xFFFF_FFFF_FFFF_FFFFLulong
0x0U .. 0xFFFF_FFFFUuint
0x1_0000_0000U .. 0xFFFF_FFFF_FFFF_FFFFUulong

An integer literal may not exceed those values.

Best Practices: Octal integer notation is not supported in 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

    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, which are ignored.


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

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


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 are reserved identifiers.

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
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 typedef (deprecated) typeid typeof
ubyte ucent uint ulong union unittest ushort
version void
wchar while with
__gshared __traits __vector __parameters

Special Tokens

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

Special Tokens
Special TokenReplaced 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
__VERSION__Compiler version as an integer
Implementation Defined: The replacement string literal for __VENDOR__ and the replacement integer value for __VERSION__.

Special Token Sequences

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