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Scanning
--------
.. glossary::
token
A single atomic unit in a Crowbar source file.
May be a :term:`keyword`, an :term:`identifier`, a :term:`constant`,
a :term:`string literal`, or a :term:`punctuator`.
Keywords, identifiers, and constants (except for :term:`character constant`\ s) must have either whitespace or a comment separating them.
Punctuators, string literals, and character constants do not require explicit separation from adjacent tokens.
keyword
One of the literal words ``bool``, :crowbar:ref:`break`, ``case``,
``char``, ``const``, ``continue``, ``default``, ``do``, ``double``,
``else``, ``enum``, ``extern``, ``float``, ``for``, ``fragile``,
``function``, ``if``, ``include``, ``int``, ``long``, ``return``,
``short``, ``signed``, ``sizeof``, ``struct``, ``switch``,
``unsigned``, ``void``, or ``while``.
identifier
A nonempty sequence of characters blah blah blah
.. todo::
figure out https://www.unicode.org/reports/tr31/tr31-33.html
constant
A numeric (or numeric-equivalent) value specified directly within the code.
May be a :term:`decimal constant`, a :term:`binary constant` , an :term:`octal constant`,
a :term:`hexadecimal constant`, a :term:`floating-point constant`, a :term:`hexadecimal floating-point constant`,
or a :term:`character constant`.
Any of these except for the character constant may contain underscores; these are ignored by the compiler and only meaningful to humans reading the code.
decimal constant
A sequence of characters matching the regular expression ``[0-9_]+``.
Denotes the numeric value of the given sequence of decimal digits.
binary constant
A sequence of characters matching the regular expression ``0[bB][01_]+``.
Denotes the numeric value of the given sequence of binary digits (after the ``0[bB]`` prefix has been removed).
octal constant
A sequence of characters matching the regular expression ``0o[0-7_]+``.
Denotes the numeric value of the given sequence of octal digits (after the ``0o`` prefix has been removed).
hexadecimal constant
A sequence of characters matching the regular expression ``0[xX][0-9a-fA-F]+``.
Denotes the numeric value of the given sequence of hexadecimal digits (after the ``0[xX]`` prefix has been removed).
floating-point constant
A sequence of characters matching the regular expression ``[0-9_]+\.[0-9_]+([eE][+-]?[0-9_]+)?``.
.. note::
Unlike in C and many other languages, ``6e3`` in Crowbar is not a valid floating-point constant.
The Crowbar-compatible spelling is ``6.0e3``.
Denotes the numeric value of the given decimal number, optionally expressed in scientific notation.
That is, ``XeY`` denotes :math:`X * 10^Y`.
hexadecimal floating-point constant
A sequence of characters matching the regular expression ``0(fx|FX)[0-9a-fA-F_]+\.[0-9a-fA-F_]+[pP][+-]?[0-9_]+``.
Denotes the numeric value of the given hexadecimal number expressed in binary scientific notation.
That is, ``XpY`` denotes :math:`X * 2^Y`.
character constant
A pair of single quotes ``'`` surrounding either a single character or an :term:`escape sequence`.
The single character may not be a single quote or a backslash ``\``.
Denotes the Unicode code point number for either the single surrounded character or the character denoted by the escape sequence.
escape sequence
One of the following pairs of characters:
* ``\'``, denoting the single quote ``'``
* ``\"``, denoting the double quote ``"``
* ``\\``, denoting the backslash ``\``
* ``\r``, denoting the carriage return (U+000D)
* ``\n``, denoting the line feed, or newline (U+000A)
* ``\t``, denoting the (horizontal) tab (U+0009)
* ``\0``, denoting a null character (U+0000)
Or a sequence of characters matching one of the following regular expressions:
* ``\\x[0-9a-fA-F]{2}``, denoting the numeric value of the given two hexadecimal digits
* ``\\x[0-9a-fA-F]{4}``, denoting the numeric value of the given four hexadecimal digits
* ``\\x[0-9a-fA-F]{8}``, denoting the numeric value of the given eight hexadecimal digits
string literal
A pair of double quotes ``"`` surrounding a sequence whose elements are either single characters or escape sequences.
No single-character element may be the double quote or the backslash.
Denotes the UTF-8-encoded sequence of bytes representing the sequence of characters which, either directly or via an escape sequence, are specified between the quotes.
punctuator
One of the literal sequences of characters ``[``, ``]``, ``(``, ``)``,
``{``, ``}``, ``.``, ``,``, ``+``, ``-``, ``*``, ``/``, ``%``, ``;``,
``!``, ``&``, ``|``, ``^``, ``~``, ``>``, ``<``, ``=``, ``->``, ``++``,
``--``, ``>>``, ``<<``, ``<=``, ``>=``, ``==``, ``!=``, ``&&``, ``||``,
``+=``, ``-=``, ``*=``, ``/=``, ``%=``, ``&=``, ``|=``, or ``^=``.
whitespace
A nonempty sequence of characters that each has a Unicode general category of either Control (``Cc``) or Separator (``Z``).
Separates tokens.
comment
Text that the compiler should ignore.
May be a :term:`line comment` or a :term:`block comment`.
line comment
A sequence of characters beginning with the characters ``//`` (outside of a :term:`string literal` or :term:`comment`) and ending with a newline character U+000A.
block comment
A sequence of characters beginning with the characters ``/*`` (outside of a :term:`string literal` or :term:`comment`) and ending with the characters ``*/``.
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