基本上我要做的是在 ANTLR 4.1 中为国际化资源标识符创建语法。到目前为止，我遇到的最困难的时间是试图让 ipv6address 的生产规则正常工作。RFC 3987中定义 ipv6address 的方式是，仅针对该生产规则，ABNF 格式基本上就有 9 种不同的替代方案：
IPv6address = 6( h16 ":" ) ls32
/ "::" 5( h16 ":" ) ls32
/ [ h16 ] "::" 4( h16 ":" ) ls32
/ [ *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32
/ [ *2( h16 ":" ) h16 ] "::" 2( h16 ":" ) ls32
/ [ *3( h16 ":" ) h16 ] "::" h16 ":" ls32
/ [ *4( h16 ":" ) h16 ] "::" ls32
/ [ *5( h16 ":" ) h16 ] "::" h16
/ [ *6( h16 ":" ) h16 ] "::"
在这里，ls32 和 h16 都是定义为的子规则：
ls32 = ( h16 ":" h16 ) / IPv4address
因此对于 h16：
h16 = 1*4HEXDIG
其中 HEXDIG 是有效十六进制数字的词法分析器规则。我尝试使用 ANTLR 语法编写此 ABNF 语法，如下所示：
grammar IRI;
iri : scheme ':' ihier_part ('?' iquery)? ('#' ifragment)? ;
ihier_part : ('//' iauthority ipath_abempty
| ipath_absolute
| ipath_rootless)?
;
iri_reference : iri
| irelative_ref
;
absolute_IRI : scheme ':' ihier_part ('?' iquery)? ;
irelative_ref : irelative_part ('?' iquery)? ('#' ifragment)? ;
irelative_part : ('//' iauthority ipath_abempty
| ipath_absolute
| ipath_noscheme)?
;
iauthority : (iuserinfo '@')? ihost (':' port)? ;
iuserinfo : (iunreserved | pct_encoded | sub_delims | ':')* ;
ihost : ip_literal
| ipv4address
| ireg_name
;
ireg_name : (iunreserved | pct_encoded | sub_delims)* ;
ipath : (ipath_abempty
| ipath_absolute
| ipath_noscheme
| ipath_rootless)?
;
ipath_abempty : ('/' isegment)* ;
ipath_absolute : '/' (isegment_nz ('/' isegment)*)? ;
ipath_noscheme : isegment_nz_nc ('/' isegment)* ;
ipath_rootless : isegment_nz ('/' isegment)* ;
isegment : (ipchar)* ;
isegment_nz : (ipchar)+ ;
isegment_nz_nc : (iunreserved | pct_encoded | sub_delims | '@')+ ;
ipchar : iunreserved
| pct_encoded
| sub_delims
| ':'
| '@'
;
iquery : (ipchar | IPRIVATE | '/' | '?')* ;
ifragment : (ipchar | '/' | '?')* ;
iunreserved : ALPHA
| DIGIT
| '-'
| '.'
| '_'
| '~'
| UCSCHAR
;
fragment
UCSCHAR : '\u00A0'..'\uD7FF' | '\uF900'..'\uFDCF' | '\uFDF0'..'\uFFEF'
| '\u40000'..'\u4FFFD' | '\u50000'..'\u5FFFD' | '\u60000'..'\u6FFFD'
| '\u70000'..'\u7FFFD' | '\u80000'..'\u8FFFD' | '\u90000'..'\u9FFFD'
| '\uA0000'..'\uAFFFD' | '\uB0000'..'\uBFFFD' | '\uC0000'..'\uCFFFD'
| '\uD0000'..'\uDFFFD' | '\uE1000'..'\uEFFFD'
;
fragment
IPRIVATE : '\uE000'..'\uF8FF' | '\uF0000'..'\uFFFFD' | '\u100000'..'\u10FFFD' ;
scheme : ALPHA (ALPHA | DIGIT | '+' | '-' | '.')* ;
port : (DIGIT)* ;
ip_literal : '[' (ipv6address | ipvFuture) ']' ;
ipvFuture : 'v' (HEXDIG)+ '.' (unreserved | sub_delims | ':')+ ;
ipv6address
locals [int i1, i2, i3, i4, i5, i6, i7, i8, i9, i10 = 0;]
: ( {$i1<=6}? h16 ':' {$i1++;} )* ls32
| '::' ( {$i2<=5}? h16 ':' {$i2++;} )* ls32
| (h16)? '::' ( {$i3<=4}? h16 ':' {$i3++;} )* ls32
| ((h16 ':')? h16)? '::' ( {$i4<=3}? h16 ':'{$i4++;} )* ls32
| (( {$i5>=0 && $i5<=2}? h16 ':' {$i5++;} )* h16)? '::' ( {$i6<=2}? h16 ':' {$i6++;} )* ls32
| (( {$i7>=0 && $i7<=3}? h16 ':' {$i7++;} )* h16)? '::' h16 ':' ls32
| (( {$i8>=0 && $i8<=4}? h16 ':' {$i8++;} )* h16)? '::' ls32
| (( {$i9>=0 && $i9<=5}? h16 ':' {$i9++;} )* h16)? '::' h16
| (( {$i10>=0 && $i10<=6}? h16 ':' {$i10++;} )* h16)* '::'
;
h16
locals [int i = 1;]
: ( {$i>=1 && $i<=4}? HEXDIG {$i++;} )* ;
ls32 : h16 ':' h16 ;
ipv4address : DEC_OCTET '.' DEC_OCTET '.' DEC_OCTET '.' DEC_OCTET ;
DEC_OCTET : '0'..'9'
| '10'..'99'
| '100'..'199'
| '200'..'249'
| '250'..'255'
;
pct_encoded : '%' HEXDIG HEXDIG ;
unreserved : ALPHA | DIGIT | '-' | '.' | '_' | '~' ;
reserved : gen_delims
| sub_delims
;
gen_delims : ':' | '/' | '?' | '#' | '[' | ']' | '@' ;
sub_delims : '!' | '$' | '&' | '\'' | '(' | ')' ;
DIGIT : [0-9] ;
HEXDIG : [0-9A-F] ;
ALPHA : [a-zA-Z] ;
WS : [' ' | '\t' | '\r' | '\n']+ -> skip ;
在我的 ANTLR 语法中，我尝试使用语义谓词来指定 ABNF 语法中定义的多重性规则，包括 ipv6address 和 h16。当我执行 org.antlr.v4.Tool 类时，我得到以下输出：
warning(125): IRI.g4:68:20: implicit definition of token 'IPRIVATE' in parser
warning(125): IRI.g4:78:4: implicit definition of token 'UCSCHAR' in parser
error(153): IRI.g4:100:0: rule 'ipv6address' contains a closure with at least one alternative that can match an empty string
warning(154): IRI.g4:40:0: rule 'ipath' contains an optional block with at least one alternative that can match an empty string
warning(154): IRI.g4:100:0: rule 'ipv6address' contains an optional block with at least one alternative that can match an empty string
warning(154): IRI.g4:100:0: rule 'ipv6address' contains an optional block with at least one alternative that can match an empty string
warning(154): IRI.g4:100:0: rule 'ipv6address' contains an optional block with at least one alternative that can match an empty string
warning(154): IRI.g4:100:0: rule 'ipv6address' contains an optional block with at least one alternative that can match an empty string
warning(154): IRI.g4:100:0: rule 'ipv6address' contains an optional block with at least one alternative that can match an empty string
warning(154): IRI.g4:100:0: rule 'ipv6address' contains an optional block with at least one alternative that can match an empty string
显然我也想摆脱警告，但我需要摆脱错误，指出“ipv6address”包含一个闭包，其中至少有一个可以匹配空字符串的替代方案。我在 StackOverflow 上看到过关于多个替代错误的类似帖子。但是，它们都没有处理可以匹配空字符串的闭包。我也很确定我将不得不将 UCSCHAR 过去 \uFFFF 中的 Unicode 字符定义为代理对，但稍后我会处理。现在只需要知道如何摆脱关闭问题。