I have a rule like this,
BLOCK_COMMENT
: ('/*' ~[!] .*? '*/' | '/**/') -> channel(HIDDEN);
But when I try to match this line,
/**/and /**/1=1
The and symbol is HIDDEN as well. Since ANTLR is greedy, it matched the last occurrence of */, and it end up with only one BLOCK_COMMENT (I was expecting two)
So, I will need something that matches not '*/', and the BLOCK_COMMENT rule should become:
'/*' then not '*/' then '*/'
Anyone know what rules can match not '*/'?
First here is a quote from the book 'Definitive ANTLR4 Reference' on the ~ operator on lexer rules:
~x Match any single character not in the set described by x . Set x
can be a single character literal, a range, or a subrule set like
~('x'|'y'|'z') or ~[xyz] .
so basically we can't use something like ~'*/'.
Since you need to interpret the comments themselfs as well, best way to do it IMHO is with lexer modes.
...
COMMENT_START : '/*' -> mode (COMMENT_MODE);
mode COMMENT_MODE;
COMMENT_END : '*/' -> mode (DEFAULT_MODE);
//match anything else that you need in this mode
...
I have assumed that you only have one mode in addition to the default one. Of course if you have more of them, you can also use popMode and pushMode.
Related
How to implement this rule in ANTLR4:
multiline-comment-text-item -> Any Unicode scalar value except /* or */
?
In ANTLR, you cannot say: "match this-or-that character, except these multiple (!) characters". You can only say "match this-or-that character, except these single (!) characters":
ANY_EXCEPT_STAR : ~[*];
ANY_EXCEPT_FSLASH : ~[/];
But doing FOO : ~[/*]; matches any single character except a / and *.
I wouldn't match multiline-comment-text-item in a lexer rule of its own, but rather inside the multiline-comment-text where it's (most likely) used:
MultilineCommentText
: '/*' .*? '*/'
;
Be sure to include the ? in there, making it ungreedy.
Note that quite often, such tokens are hidden or discarded so that they won't end up in parser rules. In that case do either:
MultilineCommentText
: '/*' .*? '*/' -> skip
;
or
MultilineCommentText
: '/*' .*? '*/' -> channel(HIDDEN)
;
See: https://github.com/antlr/antlr4/blob/master/doc/lexer-rules.md
I've just met this rule when trying to parse Swift with ANTLR4. Following is my implementation:
MULTILINE_COMMENT
: '/*' ('/'*? MULTILINE_COMMENT | ('/'* | '*'*) ~[/*])*? '*'*? '*/'
;
It's unnecessary to split multiline-comment into that many subrules as in the document.
I would like to have the following grammar (part of it):
expression
:
expression 'AND' expression
| expression 'OR' expression
| StringSequence
;
StringSequence
:
StringCharacters
;
fragment
StringCharacters
: StringCharacter+
;
fragment
StringCharacter
: ~["\]
| EscapeSequence
;
It should match things like "a b c d f" (without the quotes), as well as things like "a AND b AND c".
The problem is that my rule StringSequence is greedy, and consumes the OR/AND as well. I've tried different approaches but couldn't get my grammar to work in the correct way. Is this possible with ANTLR4? Note that I don't want to put quotes around every string. Putting quotes works fine because the rule becomes non greedy, i.e.:
StringSequence
: '"' StringCharacters? '"'
;
You have no whitespace rule so StringCharacter matches everything except quote and backslash chars (+ the escape sequenc). Include a whitespace rule to make it match individual AND/OR tokens. Additionally, I recommend to define lexer rules for string literals ('AND', 'OR') instead of embedding them in the (parser) rule(s). This way you not only get speaking names for the tokens (instead of auto generated ones) but you also can better control the match order.
Yet a naive solution:
StringSequence :
(StringCharacter | NotAnd | NotOr)+
;
fragment NotAnd :
'AN' ~'D'
| 'A' ~'N'
;
fragment NotOr:
'O' ~('R')
;
fragment StringCharacter :
~('O'|'A')
;
Gets a bit more complex with Whitespace rules. Another solution would be with semantic predicates looking ahead and preventing the read of keywords.
I am trying to write a config file grammar and get ANTLR4 to handle it. I am quite new to ANTLR (this is my first project with it).
Largely, I understand what needs to be done (or at least I think I do) for most of the config file grammar, but the files that I will be reading will have arbitrary C code inside of curly braces. Here is an example:
Something like:
#DEVICE: servo "servos are great"
#ACTION: turnRight "turning right is fun"
{
arbitrary C source code goes here;
some more arbitrary C source code;
}
#ACTION: secondAction "this is another action"
{
some more code;
}
And it could be many of those. I can't seem to get it to understand that I want to just ignore (without skipping) the source code. Here is my grammar so far:
/**
ANTLR4 grammar for practicing
*/
grammar practice;
file: (devconfig)*
;
devconfig: devid (action)+
;
devid: DEV_HDR (COMMENT)?
;
action: ACTN_HDR '{' C_BLOCK '}'
;
DEV_HDR: '#DEVICE: ' ALPHA+(IDCHAR)*
;
fragment
ALPHA: [a-zA-Z]
;
fragment
IDCHAR: ALPHA
| [0-9]
| '_'
;
COMMENT: '"' .*? '"'
;
ACTN_HDR: '#ACTION: ' ACTION_ID
;
fragment
ACTION_ID: ALPHA+(IDCHAR)*
;
C_BLOCK: WHAT DO I PUT HERE?? -> channel(HIDDEN)
;
WS: [ \t\n\r]+ -> skip
;
The problem is that whatever I put in the C_BLOCK lexer rule seems to screw up the whole thing - like if I put .*? -> channel(HIDDEN), it doesn't seem to work at all (of course, there is an error when using ANTLR on the grammar to the tune of ".*? can match the empty string" - but what should I put there if not that, so that it ignores the C code, but in such a way that I can access it later (i.e., not skipping it)?
Your C_BLOCK rule can be defined just like the usual multi line comment rule is done in so many languages. Make the curly braces part of the rule too:
C_BLOCK: CURLY .*? CURLY -> channel(HIDDEN);
If you need to nest blocks you write something like:
C_BLOCK: CURLY .*? C_BLOCK? .*? CURLY -> channel(HIDDEN);
or maybe:
C_BLOCK:
CURLY (
C_BLOCK
| .
)*?
CURLY
;
(untested).
Update: changed code to use the non-greedy kleene operator as suggested by a comment.
This is my first crack at parser generators, and, consequently ANTLR. I'm using ANTLR v4 trying to generate a simple practice parser for Morse Code with the following extra rules:
A letter (e.g., ... [the letter 's']) can be denoted as capitalized if a '^' precedes it
ex.: ^... denotes a capital 'S'
Special characters can be embeded in parentheses
ex.: (#)
Each encoded entity will be separated by whitespace
So I could encode the following sentence:
ABC a#b.com
as (with corresponding letters shown underneath):
^.- ^-... ^-.-. ( ) ._ (#) -... (.) -.-. --- --
A B C ' ' a '#' b '.' c o m
Particularly note the two following entities: ( ) (which denotes a space) and (.) (which denotes a period.
There is mainly one things that I'm finding hard to wrap my head around: The same token can take on different meanings depending on whether it is in parentheses or not. That is, I want to tell ANTLR that I want to discard whitespace, yet not in the ( ) case. Also, a Morse Code character can consist of dots-and-dashes (periods-and-dashes), yet, I don't want to consider the period in (.) as "any charachter".
Here is the grammar I have got so far:
grammar MorseCode;
file: entity*;
entity:
special
| morse_char;
special: '(' SPECIAL ')';
morse_char: '^'? (DOT_OR_DASH)+;
SPECIAL : .; // match any character
DOT_OR_DASH : ('.' | '-');
WS : [ \t\r\n]+ -> skip; // we don't care about whitespace (or do we?)
When I try it against the following input:
^... --- ...(#)
I get the following output (from grun ... -tokens):
[#0,0:0='^',<1>,1:0]
[#1,1:1='.',<4>,1:1]
...
[#15,15:14='<EOF>',<-1>,1:15]
line 1:1 mismatched input '.' expecting DOT_OR_DASH
It seems there is trouble with ambiguity between SPECIAL and DOT_OR_DASH?
It seems like your (#) syntax behaves like a quoted string in other programming languages. I would start by defining SPECIAL as:
SPECIAL : '(' .*? ')';
To ensure that . . and .. are actually different, you can use this:
SYMBOL : [.-]+;
Then you can define your ^ operator:
CARET : '^';
With these three tokens (and leaving WS as-is), you can simplify your parser rules significantly:
file
: entity* EOF
;
entity
: morse_char
| SPECIAL
;
morse_char
: CARET? SYMBOL
;
Surprise, I am building an SQL like language parser for a project.
I had it mostly working, but when I started testing it against real requests it would be handling, I realized it was behaving differently on the inside than I thought.
The main issue in the following grammar is that I define a lexer rule PCT_WITHIN for the language keyword 'pct_within'. This works fine, but if I try to match a field like 'attributes.pct_vac', I get the field having text of 'attributes.ac' and a pretty ANTLR error of:
line 1:15 mismatched character u'v' expecting 'c'
GRAMMAR
grammar Select;
options {
language=Python;
}
eval returns [value]
: field EOF
;
field returns [value]
: fieldsegments {print $field.text}
;
fieldsegments
: fieldsegment (DOT (fieldsegment))*
;
fieldsegment
: ICHAR+ (USCORE ICHAR+)*
;
WS : ('\t' | ' ' | '\r' | '\n')+ {self.skip();};
ICHAR : ('a'..'z'|'A'..'Z');
PCT_CONTAINS : 'pct_contains';
USCORE : '_';
DOT : '.';
I have been reading everything I can find on the topic. How the Lexer consumes stuff as it finds it even if it is wrong. How you can use semantic predication to remove ambiguity/how to use lookahead. But everything I read hasn't helped me fix this issue.
Honestly I don't see how it even CAN be an issue. I must be missing something super obvious because other grammars I see have Lexer rules like EXISTS but that doesn't cause the parser to take a string like 'existsOrNot' and spit out and IDENTIFIER with the text of 'rNot'.
What am I missing or doing completely wrong?
Convert your fieldsegment parser rule into a lexer rule. As it stands now it will accept input like
"abc
_ abc"
which is probably not what you want. The keyword "pct_contains" won't be matched by this rule since it is defined separately. If you want to accept the keyword in certain sequences as regular identifier you will have to include it in the accepted identifier rule.