Hi Bart, thanks very for the quick reply.
I should have made it clear that concatenating is only legal if at least one of
the operands is a string literal - that is where I am having an issue. In other
terms the input
3 3<EOF>
should cause a syntax error, while
3 "" 3<EOF>
should not (and evaluates to the string "33")
Franck
________________________________
From: Bart Kiers [via ANTLR] <ml-node+s1301665n7008016...@n2.nabble.com>
To: franck102 <franck...@yahoo.com>
Sent: Friday, November 18, 2011 1:11 PM
Subject: Re: String concatenation expression rule
On Fri, Nov 18, 2011 at 12:39 PM, franck102 <[hidden email]> wrote:
> I am writing a grammar for a fairly complex expression language, and in
> particular I need to support string concatenation which is performed simply
> by separating string literals with a space; and which automatically
> converts
> other expressions to a string if needed to concatenate:
> "a" "b" -> "ab"
> 2+3 "mm" -> "5mm"
>
> I suspect I could use predicates to write a rule like this:
>
> concatExpression
> : ( expression | STRING_LITERAL )+ { apply only if at least
> one of the elements is a string literal }?
>
> Is there a way to achieve this? The alternative formulations I can think of
> are pretty messy...
>
> As far as I understand it, you don't need any predicate. I see a
concat-expression has a lower precedence than addition, in which case this
could do the trick:
grammar T;
options {
output=AST;
}
tokens {
ROOT;
CONCAT;
}
parse
: (expression ';')* EOF -> ^(ROOT expression*)
;
expression
: (add -> add) (add+ -> ^(CONCAT add+))?
;
add
: atom (('+' | '-')^ atom)*
;
atom
: Number
| String
| '(' expression ')' -> expression
;
Number : '0'..'9'+ ('.' '0'..'9'+)?;
String : '"' ~'"'* '"';
Space : ' ' {skip();};
You can test it with the following class:
import org.antlr.runtime.*;
import org.antlr.runtime.tree.*;
import org.antlr.stringtemplate.*;
public class Main {
public static void main(String[] args) throws Exception {
String src = "42 - 2; 2 + 3 \"mm\"; \"a\" \"b\" 4-3-2 \"c\"; \"pi = \"
3.14159;";
TLexer lexer = new TLexer(new ANTLRStringStream(src));
TParser parser = new TParser(new CommonTokenStream(lexer));
CommonTree root = (CommonTree)parser.parse().getTree(); ;
System.out.println(new DOTTreeGenerator().toDOT(root));
}
}
Regards,
Bart.
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