Re: [Flex-help] How to Match 0 or More Whitespace at Line Beginning?

[Lene N. <neu...@gm...>]

Sorry, I made a mistake, but it seems flex could match tokens of 0
character.
Thanks a lot!

2010/12/30 John P. Hartmann <jph...@gm...>

> ^[ ]+    return INDENTED;
> ^[^ \n]+  yylval.string=strdup(yytext); return NOTINDENTEDWORD.
>
> You could also match a null string, but you'd need start conditions to
> avoid matching it forever:
>
> ^""/[^ \n]  return ...
>
> And don't forget to exclude newlines when you are excluding blanks.
>
> On 30 December 2010 07:00, Lene Neuron <neu...@gm...> wrote:
> > Hi
> >
> > Thanks for your advice, but it seems that rule
> >    ^/[^ ] { return NO_INDENT; }
> > doesn't work. Flex still prints the message ``unrecognized rule''.
> >
> >    I guess flex will not match a token of 0 character, so the following
> > rule
> >    a* { printf("%d\n", yyleng); }
> > has the same effect as
> >    a+ { printf("%d\n", yyleng); }
> >
> >    I have bypass this problem in syntax analysis, by defining ``indent''
> as
> > indent
> > : INDENT { return yyleng; }
> > | { return 0; }
> > ;
> >
> > Neuron Teckid
> >
> > 2010/12/30 Alexandre Bique <biq...@gm...>
> >
> >> On Thu, Dec 30, 2010 at 3:09 AM, Lene Neuron <neu...@gm...>
> >> wrote:
> >> > Hi, all:
> >> > I would like to use *flex* to parse *Python* source code. The problem
> is
> >> > that I cannot match 0 whitespace character at line beginning, which
> >> > indicates the line is not indented.
> >> >
> >> >    I have tried this at first:
> >> >
> >> > *^[ ]* { return INDENT; }*
> >> >
> >> > but it didn't work if no whitespace at line beginning. Then I changed
> it
> >> as
> >> >
> >> > *^[ ]+ { return INDENT; }*
> >> > *^     { return INDENT; }*
> >> >
> >> > but I was told that the latter rule is an "unrecognized rule".
> >> >
> >> >    Would you please give me some ideas? Thanks in advance.
> >> >
> >> > Neuron Teckid
> >>
> >> Hi,
> >>
> >> I am not sure, but you can do something like that :
> >>
> >> ^/[^ ] { return NO_INDENT; }
> >>
> >> See the explanation from the manual:
> >>
> >> Chapter 6 "Patterns":
> >>
> >> [...]
> >>
> >> `r/s'
> >>     an `r' but only if it is followed by an `s'.  The text matched by
> >>     `s' is included when determining whether this rule is the longest
> >>     match, but is then returned to the input before the action is
> >>     executed.  So the action only sees the text matched by `r'.  This
> >>     type of pattern is called "trailing context".  (There are some
> >>     combinations of `r/s' that flex cannot match correctly. *Note
> >>     Limitations::, regarding dangerous trailing context.)
> >>
> >> `^r'
> >>     an `r', but only at the beginning of a line (i.e., when just
> >>     starting to scan, or right after a newline has been scanned).
> >>
> >> [...]
> >>
> >> But, from my point of view, it should be better to count the
> >> indentation level rather than match the "no indentation" case.
> >> You can also have multiple states in your lexer (see chapter 10 "Start
> >> Conditions").
> >>
> >> Good luck.
> >>
> >> --
> >> Alexandre Bique
> >>
> >
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