CLHS: Section 2.1.4.2

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## 2.1.4.2 Constituent Traits

Every character has one or more constituent traits that define how the character is to be interpreted by the Lisp reader when the character is a constituent character. These constituent traits are alphabetic[2], digit, package marker, plus sign, minus sign, dot, decimal point, ratio marker, exponent marker, and invalid. Figure 2-8 shows the constituent traits of the standard characters and of certain semi-standard characters; no mechanism is provided for changing the constituent trait of a character. Any character with the alphadigit constituent trait in that figure is a digit if the current input base is greater than that character's digit value, otherwise the character is alphabetic[2]. Any character quoted by a single escape is treated as an alphabetic[2] constituent, regardless of its normal syntax.

    constituent  traits          constituent  traits
    character                    character
    ----------

    Backspace    invalid         {            alphabetic[2]
    Tab          invalid*        }            alphabetic[2]
    Newline      invalid*        +            alphabetic[2], plus sign
    Linefeed     invalid*        -            alphabetic[2], minus sign
    Page         invalid*        .            alphabetic[2], dot, decimal point
    Return       invalid*        /            alphabetic[2], ratio marker
    Space        invalid*        A, a         alphadigit
    !            alphabetic[2]   B, b         alphadigit
    "            alphabetic[2]*  C, c         alphadigit
    #            alphabetic[2]*  D, d         alphadigit, double-float exponent marker
    $            alphabetic[2]   E, e         alphadigit, float exponent marker
    %            alphabetic[2]   F, f         alphadigit, single-float exponent marker
    &            alphabetic[2]   G, g         alphadigit
    '            alphabetic[2]*  H, h         alphadigit
    (            alphabetic[2]*  I, i         alphadigit
    )            alphabetic[2]*  J, j         alphadigit
    *            alphabetic[2]   K, k         alphadigit
    ,            alphabetic[2]*  L, l         alphadigit, long-float exponent marker
    0-9          alphadigit      M, m         alphadigit
    :            package marker  N, n         alphadigit
    ;            alphabetic[2]*  O, o         alphadigit
    <            alphabetic[2]   P, p         alphadigit
    =            alphabetic[2]   Q, q         alphadigit
    >            alphabetic[2]   R, r         alphadigit
    ?            alphabetic[2]   S, s         alphadigit, short-float exponent marker
    @            alphabetic[2]   T, t         alphadigit
    [            alphabetic[2]   U, u         alphadigit
    \            alphabetic[2]*  V, v         alphadigit
    ]            alphabetic[2]   W, w         alphadigit
    ^            alphabetic[2]   X, x         alphadigit
    _            alphabetic[2]   Y, y         alphadigit
    `            alphabetic[2]*  Z, z         alphadigit
    |            alphabetic[2]*  Rubout       invalid
    ~            alphabetic[2]

Figure 2-8. Constituent Traits of Standard Characters and Semi-Standard Characters

The interpretations in this table apply only to characters whose syntax type is constituent. Entries marked with an asterisk (*) are normally shadowed[2] because the indicated characters are of syntax type whitespace[2], macro character, single escape, or multiple escape; these constituent traits apply to them only if their syntax types are changed to constituent.

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