> 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 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)

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 also factor the task into smaller pieces:

   sign = char '+' <|> char '-'

   exponent = do
     char 'E'
     option sign
     xs <- many1 digit
     if length xs > 3 then fail else return ()

   number = do
     option sign
     many1 digit
     option (char '.')
     many1 digit
     option exponent

You can also do things like write a function that builds a special kind 
of parser given a simpler spec.

With a regex, on the other hand, you cannot even statically guarantee 
that a given string is even a syntactically valid regex. And that's 
before you try to programmatically construct new ones. :-P

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