Re: Regular expressions in lexing and parsing

On 21/05/2019 8:14 AM, Hans-Peter Diettrich wrote:
> ...
> OTOH I wonder how a formal (regex?) FORTRAN lexer grammar could look like?
>
> DoDi


IMHO, statement classification with a DFA isn't technically feasible
in fixed form due to the labeled DO statements. In all other cases I
think that a ")=" or "]=" outside a literal character constant implies
an assignment statement, which can be matched by a DFA.


In addition, fixed form requires some filtering for spaces and each
form distinct filtering for continuation patterns. Of course, there are
more details. That explained see ie the file OMP_Fortran_Scanner.txt at:
https://github.com/drikosev/Fortran


Ev. Drikos


-----------------------------------------------------------------------
PS: still, the generated tool fails ie to parse the following code:


$ cat doi.f
                do 10 J =(((((1))))),2
                do 10 (I)=1,2
10 print *, "hello world"
                end






> [I've written Fortran 77 parsers.  It was quite context sensitive,
> particularly
> deciding whether a statement was an assignment or something else. e.g.
>
> FORMAT(I4) = 42
      Still, we've the hard to parse optional commas in format items.


> DO 10 I = 1.23   (oops)
>
> Modern Fortran no longer ignores spaces which makes lexing a lot easier.
> -John]

Return to the comp.compilers page.
Search the comp.compilers archives again.