Invisible wrote:

> - There can be *zero* or more characters before the decimal point. (But 
> notice that there must be more than zero characters *in total* before 
> and after the decimal point. It's just that there can be zero in either 
> place, but not both.)

A quickie C# example ...

Code:


     class Program
     {

         static string[] tokens = new string[] {"0.", ".0", ".", "1.1", 
"1.1.1", "1e1", "1x1", "s1", "1s"};

         static void Main(string[] args)
         {

             Regex rx = new 
Regex(@"^[+-]?(([0-9]*(\.[0-9]+)?)|([0-9+]\.))(e[0-9]+)?$");

             foreach(string t in tokens)
             {
                 if(rx.Match(t).Success)
                 {
                     Console.WriteLine("\"{0}\" --> Real", t);
                 }
                 else
                 {
                     Console.WriteLine("\"{0}\" --> Identifier", t);
                 }
             }

             System.Console.ReadKey();
         }


Output:

"0." --> Real
".0" --> Real
"." --> Identifier
"1.1" --> Real
"1.1.1" --> Identifier
"1e1" --> Real
"1x1" --> Identifier
"s1" --> Identifier
"1s" --> Identifier

The relavent portion is this regex:

^[+-]?(([0-9]*(\.[0-9]+)?)|([0-9+]\.))(e[0-9]+)?$

Which does what Warp describes, but also takes care of the case with .0 
and 0.

> - The "e" can also be "E".

you can easily replace the e for exponent with [eE], which allows either 
lower or upper case. :)

> - The exponent is an integer, not a real.
> 
> See? Not as easy as it looks, is it? Gotta pay careful attention to 
> *exactly* what the manual says is and isn't permissible.
> 
>>   In an actual BNF-style parser the rule probably becomes simpler because
>> the repetition can be removed.
> 
> It would be *really nice* if the reference manual included a BNF syntax 
> diagram... :-S
> 
> Between the rules for splitting up tokens, the tricky rules for escaping 
> things in strings, and characters that have multiple meanings depending 
> on context, it's really quite hard!
> 
> E.g., "<" is the start of a string, "<~" is the start of another string, 
> and "<<" is an ordinary name object. Go figure. Similarly, "[" is 
> classified as a "delimiter character", yet it's also the *name* of an 
> operator (and a name is a sequence of "regular characters" - that is, 
> can't contain delimiters).
> 
> It all gets confusing very fast...


-- 
~Mike

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