How Lexer lookahead works with greedy and non-greedy matching in ANTLR3 and ANTLR4?

问题

If someone would clear my mind from the confusion behind look-ahead relation to tokenizing involving greery/non-greedy matching i'd be more than glad. Be ware this is a slightly long post because it's following my thought process behind.

I'm trying to write antlr3 grammar that allows me to match input such as:

"identifierkeyword"

I came up with a grammar like so in Antlr 3.4:

KEYWORD: 'keyword' ;

IDENTIFIER
: 
  (options {greedy=false;}: (LOWCHAR|HIGHCHAR))+ 
;

/** lowercase letters */
fragment LOWCHAR
:   'a'..'z';
/** uppercase letters */
fragment HIGHCHAR
:   'A'..'Z';

parse: IDENTIFIER KEYWORD EOF;

however it complains about it can never match IDENTIFIER this way, which i don't really understand. (The following alternatives can never be matched: 1)

Basically I was trying to specify for the lexer that try to match (LOWCHAR|HIGHCHAR) non-greedy way so it stops at KEYWORD lookahead. What i've read so far about ANTLR lexers that there supposed to be some kind of precedence of the lexer rules. If i specify KEYWORD lexer rule first in the lexer grammar, any lexer rules that come after shouldn't be able to match the consumed characters.

After some searching I understand that problem here is that it can't tokenize the input the right way because for example for input: "identifierkeyword" the "identifier" part comes first so it decides to start matching the IDENTIFIER rule when there is no KEYWORD tokens matched yet.

Then I tried to write the same grammar in ANTLR 4, to test if the new run-ahead capabilities can match what i want, it looks like this:

KEYWORD: 'keyword' ;

/** lowercase letters */
fragment LOWCHAR
:   'a'..'z';
/** uppercase letters */
fragment HIGHCHAR
:   'A'..'Z';

IDENTIFIER
: 
  (LOWCHAR|HIGHCHAR)+?
;

parse: IDENTIFIER KEYWORD EOF;

for the input: "identifierkeyword" it produces this error: line 1:1 mismatched input 'd' expecting 'keyword'

it matches character 'i' (the very first character) as an IDENTIFIER token, and then the parser expects a KEYWORD token which he doesn't get this way.

Isn't the non-greedy matching for the lexer supposed to match till any other possibility is available in the look ahead? Shouldn't it look ahead for the possibility that an IDENTIFIER can contain a KEYWORD and match it that way?

I'm really confused about this, I have watched the video where Terence Parr introduces the new capabilities of ANTLR4 where he talks about run-ahead threads that watch for all "right" solutions till the end while actually matching a rule. I thought it would work for Lexer rules too, where a possible right solution for tokenizing input "identifierkeyword" is matching IDENTIFIER: "identifier" and matching KEYWORD: "keyword"

I think I have lots of wrongs in my head about non-greedy/greedy matching. Could somebody please explain me how it works?

After all this I've found a similar question here: ANTLR trying to match token within longer token and made a grammar corresponding to that:

parse
:   
  identifier 'keyword'
;

identifier
:   
  (HIGHCHAR | LOWCHAR)+
;

/** lowercase letters */
LOWCHAR
:   'a'..'z';
/** uppercase letters */
HIGHCHAR
:   'A'..'Z';

This does what I want now, however I can't see why I can't change the identifier rule to a Lexer rule and LOWCHAR and HIGHCHAR to fragments. A Lexer doesn't know that letters in "keyword" can be matched as an identifier? or vice versa? Or maybe it is that rules are only defined to have a lookahead inside themselves, not all possible matching syntaxes?

回答1:

The easiest way to resolve this in both ANTLR 3 and ANTLR 4 is to only allow IDENTIFIER to match a single input character, and then create a parser rule to handle sequences of these characters.

identifier : IDENTIFIER+;
IDENTIFIER : HIGHCHAR | LOWCHAR;

This would cause the lexer to skip the input identifier as 10 separate characters, and then read keyword as a single KEYWORD token.

The behavior you observed in ANTLR 4 using the non-greedy operator +? is similar to this. This operator says "match as few (HIGHCHAR|LOWCHAR) blocks as possible while still creating an IDENTIFIER token". Clearly the fewest number to create the token is one, so this was effectively a highly inefficient way of writing IDENTIFIER to match a single character. The reason the parse rule failed to handle this is it only allows a single IDENTIFIER token to appear before the KEYWORD token. By creating a parser rule identifier like I showed above, the parser would be able to treat sequences of IDENTIFIER tokens (which are each a single character), as a single identifier.

Edit: The reason you get the message "The following alternatives can never be matched..." in ANTLR 3 is the static analysis has determined that the positive closure in the rule IDENTIFIER will never match more than 1 character because the rule will always be successful with exactly 1 character.

来源：https://stackoverflow.com/questions/21467473/how-lexer-lookahead-works-with-greedy-and-non-greedy-matching-in-antlr3-and-antl