Simple Xtext example generates grammar that Antlr4 doesn't like - who's to blame? -


while using xtext, have come across problem , not sure if antlr4 or xtext @ fault or if i'm missing something. understand antlr4 not supported xtext, seems particular case should not cause problem.

here simple xtext file: 

grammar com.github.jsculley.antlr4.test org.eclipse.xtext.common.terminals generate test "http://www.github.com/jsculley/antlr4/test" arule:     name=string ;

string defined in xtext rule org.eclipse.xtext.common.terminals:

terminal string :              '"' ( '\\' . /* 'b'|'t'|'n'|'f'|'r'|'u'|'"'|"'"|'\\' */ | !('\\'|'"') )* '"' |             "'" ( '\\' . /* 'b'|'t'|'n'|'f'|'r'|'u'|'"'|"'"|'\\' */ | !('\\'|"'") )* "'"         ;

the generated antlr grammar has following rule:

rule_string : ('"' ('\\' .|~(('\\'|'"')))* '"'|'\'' ('\\' .|~(('\\'|'\'')))* '\'');

the antlr 3.5.2 tool has no problem rule, antlr4 tool spits out following errors:

error(50): internaltest.g:102:29: syntax error: '(' came complete surprise me while looking lexer rule element error(50): internaltest.g:102:62: syntax error: '(' came complete surprise me while looking lexer rule element error(50): internaltest.g:102:74: syntax error: mismatched input ')' expecting semi while matching lexer rule error(50): internaltest.g:106:25: syntax error: '(' came complete surprise me while looking lexer rule element error(50): internaltest.g:106:36: syntax error: mismatched input ')' expecting semi while matching lexer rule

antlr4 doesn't (and seemingly uneccessary) sets of parentheses around group after each '~' operator. question is, xtext generating bad grammar, or antlr4 not handling valid construct?

it seems antlr 4 not handle parenthesis correctly: parser issues mutual left recursion error when left-recursive part of rule in parenthesis.

so, remove useless parenthesis , antlr 4 should generate anltr 3 compatible parser. ported pl/sql grammar antlr 3 -> antlr 4. moreover, anltr 4 have more powerfull parsing algorithm compare previous version.


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