Matching empty strings in parser

[vignesh kailas]

I have a grammar like this in antlr ver4:

csrc_includes :

   (header)*

    ;

header : HASH INCLUDE_LITERAL ANGULAR_OPEN FILE_NAME ANGULAR_CLOSE

The above grammar is throwing an error stating that the csrc_includes can evaluate to a null expression as well. Now, I do understand the implication of this grammar and the meaning of the error. But is there a way that will help me define a grammar like this (implies that there can be no header includes inside a c src file).

Big thanks in advance :)

[Mike Lischke]

Make this rule optional in the calling context and use this loop for it:

csrc_includes:
header+
;

Mike
--
www.soft-gems.net

[Kevin Cummings]

On 03/03/16 04:05, vignesh kailas wrote:
> I have a grammar like this in antlr ver4:
>
> csrc_includes :
> (header)*
> ;
>
> header : HASH INCLUDE_LITERAL ANGULAR_OPEN FILE_NAME ANGULAR_CLOSE

First of all., csrc_includes and header are parser rules. Shouldn't
they instead be lexer rules?

Second, instead of using csrc_includes in your grammar, could you
instead use:

CSRC_INCLUDES : (HEADER)+
;

Then, instead of referencing CSRC_INCLUDES in your grammar, you could
replace it with references to (CSRC_INCLUDES)? Make their use
optional, but if used require one or more includes to be present.

> The above grammar is throwing an error stating that the csrc_includes
> can evaluate to a null expression as well. Now, I do understand the
> implication of this grammar and the meaning of the error. But is there a
> way that will help me define a grammar like this (implies that there can
> be no header includes inside a c src file).
>
> Big thanks in advance :)
>

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Kevin J. Cummings
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[vignesh kailas]

Thanks guys :)