[antlr-interest] How can I insert an action when a su-brule fails as well as when this is not present?

[Gabriel Petrovay]

Hi,

I would like to perform a custom action when a subrule fails (=does not
match).

For example:
r : sr1 { /* action when sr1 is matched */ } (sr2)?

I would like an action to be executed when the subrule 2 does not match. I
already tries to make a new rule out of sr2 like this:

r : sr1 { /* action when sr1 is matched */ } (sr2)?
sr2 : 'KW' ss1 ;

As you can see in my case sr2 is prefixed with the keyword 'KW'. ANTLR
generates for this in the code or rule r an if expression to check if after
matching sr1 the keyword 'KW' is next. If not, sr2 if not even called. This
is what I want to capture. The missing sr2. Are there any other solutions
besides using a debug token stream which I want to avoid.

Thanks!
Regards,
Gabriel

-- 
MSc Gabriel Petrovay
MCSA, MCDBA, MCAD
Mobile: +41(0)787978034
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