Looking at the grammar at http://www.adahome.com/rm95/rm9x-P.html
and http://www.adaic.org/resources/add_content/standards/05rm/html/RM-2-4.html
I see the following restrictions apply:
- A number must begin and end with a digit. There must be at least one
digit on either side of each underscore and decimal point.
- There may be at most one decimal point in a number.
- The base must be between 2 and 16, inclusive.
[-+]?(
[0-9](_?[0-9])*([.][0-9](_?[0-9])*)?|
([2-9]|1[0-6])#[0-9a-fA-F](_?[0-9a-fA-F])*([.][0-9a-fA-F](_?[0-9a-fA-F])*)?#
)([eE][+-]?[0-9](_?[0-9])*)?
If you want a slightly smaller version, we can drop the "only one decimal
point" rule and get
[-+]?(
[0-9]([._]?[0-9])*|
([2-9]|1[0-6])#[0-9a-fA-F]([._]?[0-9a-fA-F])*#
)([eE][+-]?[0-9](_?[0-9])*)?
Ideally, for a based number, the range of acceptable digits would
depend on the base. Which is possible for a finite-state machine since
the number of bases is finite, but laeds to a bit of a state explosion.
Here's an approximation that separates bases 2-9 and 10-16:
[-+]?(
[0-9](_?[0-9])*([.][0-9](_?[0-9])*)?|
[2-9]#[0-8](_?[0-8])*([.][0-8](_?[0-8])*)?#|
1[0-6]#[0-9a-fA-F](_?[0-9a-fA-F])*([.][0-9a-fA-F](_?[0-9a-fA-F])*)?#
)([eE][+-]?[0-9](_?[0-9])*)?
because I think that this is overdoing it:
[-+]?(
[0-9](_?[0-9])*([.][0-9](_?[0-9])*)?|
2#[0-1](_?[0-1])*([.][0-1](_?[0-1])*)?#|
3#[0-2](_?[0-2])*([.][0-2](_?[0-2])*)?#|
4#[0-3](_?[0-3])*([.][0-3](_?[0-3])*)?#|
5#[0-4](_?[0-4])*([.][0-4](_?[0-4])*)?#|
6#[0-5](_?[0-5])*([.][0-5](_?[0-5])*)?#|
7#[0-5](_?[0-5])*([.][0-5](_?[0-5])*)?#|
8#[0-7](_?[0-7])*([.][0-7](_?[0-7])*)?#|
9#[0-8](_?[0-8])*([.][0-8](_?[0-8])*)?#|
10#[0-9](_?[0-9])*([.][0-9](_?[0-9])*)?#|
11#[0-9aA](_?[0-9aA])*([.][0-9aA](_?[0-9aA])*)?#|
12#[0-9abAB](_?[0-9abAB])*([.][0-9abAB](_?[0-9abAB])*)?#|
13#[0-9a-cA-C](_?[0-9a-cA-C])*([.][0-9a-cA-C](_?[0-9a-cA-C])*)?#|
14#[0-9a-dA-D](_?[0-9a-dA-D])*([.][0-9a-dA-D](_?[0-9a-dA-D])*)?#|
15#[0-9a-eA-E](_?[0-9a-eA-E])*([.][0-9a-eA-E](_?[0-9a-eA-E])*)?#|
16#[0-9a-fA-F](_?[0-9a-fA-F])*([.][0-9a-fA-F](_?[0-9a-fA-F])*)?#|
)([eE][+-]?[0-9](_?[0-9])*)?
Another point is that Ada doesn't actually include leading + or -
signs in the syntax for "number", but rather makes them unary operators.
This means that spaces are allowed, and whether you want to include them
in the "number" pattern is a judgement call.
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