Shift/Reduce conflict in bison - c++

Question

I was trying to do a simple parsing of general html codes.
Here's my entire bison file (example4.y).
%{
#include <iostream>
#include <cstring>
using namespace std;
extern "C" int yylex();
extern "C" int yyparse();
extern "C" FILE *yyin;
void yyerror(const char *str)
{
cout<<"Error: "<<str<<"\n";
}
int yywrap()
{
return 0;
}
main()
{
yyparse();
}
%}
%token NUMBER LANGLE CLOSERANGLE RANGLE SLASH ANYTHING
%union
{
int intVal;
float floatVal;
char *strVal;
}
%%
tag: |
opening_tag anything closing_tag
{
if(strcmp($<strVal>1,$<strVal>3)==0){
cout<<"\n[i] Tag Matches: "<<$<strVal>1;
cout <<"\n[!] The text: "<<$<strVal>2;
} else {
cout<<"\n[!] Tag Mismatch: "<<$<strVal>1<<" and "<<$<strVal>3;
}
$<strVal>$ = $<strVal>2;
}
|
opening_tag tag closing_tag
{
if(strcmp($<strVal>1,$<strVal>3)==0){
cout<<"\n[i] Tag Matches: "<<$<strVal>1;
cout <<"\n[!] The text: "<<$<strVal>2;
} else {
cout<<"\n[!] Tag Mismatch: "<<$<strVal>1<<" and "<<$<strVal>3;
}
}
;
opening_tag:
LANGLE ANYTHING RANGLE
{
$<strVal>$ = $<strVal>2;
}
;
anything:
ANYTHING
{
$<strVal>$ = $<strVal>1;
}
;
closing_tag:
LANGLE SLASH ANYTHING RANGLE
{
$<strVal>$= $<strVal>3;
}
%%
The error i get is: example4.y: conflicts: 1 shift/reduce
I think it has to do something with opening_tag tag closing_tag but i could not think what's happening here?
Any help?

Answer 1

It's because of two rules that start with opening_tag. The parser has to decide between the rules by lookint at most one token ahead, but it cannot. <FOO> may lead to either rule, and this requires two more tokens of lookahead.
You can do this:
tag : /* nothing */
| opening_tag contents closing_tag
;
contents: tag
| anything
;
UPDATE This new grammar has a different shift/reduce conflict. (UPDATE2: or perhaps it's the same one). Because a tag can be empty, the parser cannot decide what to do at this input:
<Foo> <...
^
|
input is here
If the next symbol is a slash, then we have a closing tag, and the empty tag rule should be matched. If the next symbol is not a slash, then we have an opening tag, and the non-empty tag rule should be matched. But the parser cannot know, it is only allowed to look at <.
The solution would be to create a new token, LANGLE_SLASH, for the </ combination.

Answer 2

The problem is that tag can be empty, so that <x>< might be the beginning of opening_tag tag closing_tag or of opening_tag opening_tag. Consequently, bison cannot tell whether to reduce an empty tag before shifting the <.
You should be able to fix it by removing the empty production for tag and adding an explicit production for opening_tag closing_tag.