Invisible wrote:
>> OK, so how do you do
>>
>> (\+|-)[0-9]+(\.[0-9]+)?(E(\+|-)?[0-9]{1,3})?
>>
>> in your parser language?
> 
> OK, that's one big ol' complex regex, right there.

That's a pretty simple regex, actually.

>> (That is, optional sign, one or more digits, optional decimal point
>> followed by one or more digits, optional E followed by optional sign
>> followed by one to three digits.)
> 
> If I've understood the spec correctly, it's
> 
>   do
>     option (char '+' <|> char '-')
>     many1 digit
>     option (char '.')
>     many1 digit
>     option (do char 'E'; option (char '+' <|> char '-'); many1 digit)

Bzzzt. Sorry. That requires at least a 2-digit number.

> Enforcing that the exponent is less than or equal to 3 digits would be 
> slightly more wordy. The obvious way is
> 
>   xs <- many1 digit
>   if length xs > 3 then fail else return ()
> 
> Notice that since this is written in a /real/ programming language and 
> not a text string, we can do
> 
>   sign = char '+' <|> char '-'
> 
>   number = do
>     option sign
>     many1 digit
>     option (char '.')
>     many1 digit
>     option (do char 'E'; option sign; many1 digit)
> 
> and save a little typing.

You can do that with many regexp engines too. That's what I meant by 
"trivial macros".

> You can also factor the task into smaller pieces:

Yeah, that's just a text substitution in the regexp expression.

> With a regex, on the other hand, you cannot even statically guarantee 
> that a given string is even a syntactically valid regex. 

Nonsense. How'd you get that?

If you mean you can't use a regexp to parse a regexp, well, yeah, so?

-- 
Darren New, San Diego CA, USA (PST)
   Serving Suggestion:
     "Don't serve this any more. It's awful."

Post a reply to this message