#### 2.3.1.1 Potential Numbers as Tokens ¶

To allow implementors and future Common Lisp standards to extend the syntax of numbers, a syntax for potential numbers is defined that is more general than the syntax for numbers. A token is a potential number if it satisfies all of the following requirements:

1.

  The token consists entirely of digits, signs, ratio markers, decimal points (`.`), extension characters (`^` or `_`), and number markers. A number marker is a letter. Whether a letter may be treated as a number marker depends on context, but no letter that is adjacent to another letter may ever be treated as a number marker. Exponent markers are number markers.

2.

  The token contains at least one digit. Letters may be considered to be digits, depending on the current input base, but only in tokens containing no decimal points.

3.

  The token begins with a digit, sign, decimal point, or extension character,

  [Reviewer Note by Barmar: This section is unnecessary because the first bullet already omits discussion of a colon (package marker).] but not a package marker. The syntax involving a leading package marker followed by a potential number is not well-defined. The consequences of the use of notation such as `:1`, `:1/2`, and `:2^3` in a position where an expression appropriate for read is expected are unspecified.

4.

  The token does not end with a sign.

If a potential number has number syntax, a number of the appropriate type is constructed and returned, if the number is representable in an implementation. A number will not be representable in an implementation if it is outside the boundaries set by the implementation-dependent constants for numbers. For example, specifying too large or too small an exponent for a float may make the number impossible to represent in the implementation. A ratio with denominator zero (such as `-35/000`) is not represented in any implementation. When a token with the syntax of a number cannot be converted to an internal number, an error of type reader-error is signaled. An error must not be signaled for specifying too many significant digits for a float; a truncated or rounded value should be produced.

If there is an ambiguity as to whether a letter should be treated as a digit or as a number marker, the letter is treated as a digit.