[antlr-interest] comment must be preceded by whitespace

[mzukowski at yci.com]

You are running into linear approximation lookahead issues.  

I would recommend setting a flag when you hit a newline and unsetting it
after the first non-whitespace character.  If the flag is set and you see
'/' then it is a comment.  If not it's just a '/'.

The easiest way to deal with the flag is to set in in the newline() function
and unset it in match() when match is called on a non-WS character.

Monty

-----Original Message-----
From: charlesskelton [mailto:c.skelton at skelton.de] 
Sent: Wednesday, March 17, 2004 3:46 PM
To: antlr-interest at yahoogroups.com
Subject: [antlr-interest] comment must be preceded by whitespace

I'm new to antlr.

I'm trying to write lexer for a language, k, in which single line 
comments begin with a '/'. If there is no whitespace before the '/', 
or the '/' is not in column 1, then it is not a comment.

e.g

/this is a comment
   /this is also a comment
a:+/!10 /this text after the second forward slash is a comment

In this language, +/!10 actually means sum over numbers 0 thru 9, 
but that is not very relevant here, except it shows the other use 
of '/'.

I did try to define the comment as

SL_COMMENT
	:
            ((" /")  (~('\n'|'\r'))* ('\n'|'\r'('\n')?))
         |  (("\t/") (~('\n'|'\r'))* ('\n'|'\r'('\n')?))
         |  (("\f/") (~('\n'|'\r'))* ('\n'|'\r'('\n')?))
         |  {getColumn()==1}? (('/')  (~('\n'|'\r'))* 
('\n'|'\r'('\n')?))
	;

but it complains when the first char is not a '/'. I would define it 
as

          (("\n/")  (~('\n'|'\r'))* ('\n'|'\r'('\n')?))

but I need \n to match WS type.

Any help would be greatly appreciated.

Many thanks,
Charlie





 
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