antlr4: Grammar ambiguity, left-recursion, both? - antlr4

Question

My grammar, shown below, does not compile. The returned error (from the antlr4 maven plugin) is:
[INFO] --- antlr4-maven-plugin:4.3:antlr4 (default-cli) # beebell ---
[INFO] ANTLR 4: Processing source directory /Users/kodecharlie/workspace/beebell/src/main/antlr4
[INFO] Processing grammar: DateRange.g4
org\antlr\v4\parse\GrammarTreeVisitor.g: node from line 13:87 mismatched tree node: startTime expecting <UP>
org\antlr\v4\parse\GrammarTreeVisitor.g: node from after line 13:87 mismatched tree node: RULE expecting <UP>
org\antlr\v4\parse\GrammarTreeVisitor.g: node from line 13:87 mismatched tree node: startTime expecting <UP>
org\antlr\v4\parse\GrammarTreeVisitor.g: node from after line 13:87 mismatched tree node: RULE expecting <UP>
org\antlr\v4\parse\GrammarTreeVisitor.g: node from line 13:87 mismatched tree node: startTime expecting <UP>
org\antlr\v4\parse\GrammarTreeVisitor.g: node from after line 13:87 mismatched tree node: RULE expecting <UP>
org\antlr\v4\parse\GrammarTreeVisitor.g: node from line 13:87 mismatched tree node: startTime expecting <UP>
org\antlr\v4\parse\GrammarTreeVisitor.g: node from after line 13:87 mismatched tree node: RULE expecting <UP>
[ERROR] error(20): internal error: Rule HOUR undefined
[ERROR] error(20): internal error: Rule MINUTE undefined
[ERROR] error(20): internal error: Rule SECOND undefined
[ERROR] error(20): internal error: Rule HOUR undefined
[ERROR] error(20): internal error: Rule MINUTE undefined
I can see how the grammar might be confused -- Eg, whether 2 digits is a MINUTE, SECOND, or HOUR (or maybe the start of a year). But a few articles suggest this error results from left-recursion.
Can you tell what's going on?
Thanks. Here's the grammar:
grammar DateRange;
range : startDate (THRU endDate)? | 'Every' LONG_DAY 'from' startDate THRU endDate ;
startDate : dateTime ;
endDate : dateTime ;
dateTime : GMTOFF | SHRT_MDY | YYYYMMDD | (WEEK_DAY)? LONG_MDY ;
// Dates.
GMTOFF : YYYYMMDD 'T' HOUR ':' MINUTE ':' SECOND ('-'|'+') HOUR ':' MINUTE ;
YYYYMMDD : YEAR '-' MOY '-' DOM ;
SHRT_MDY : MOY ('/' | '-') DOM ('/' | '-') YEAR ;
LONG_MDY : (SHRT_MNTH '.'? | LONG_MNTH) WS DOM ','? (WS YEAR (','? WS TIMESPAN)? | WS startTime)? ;
YEAR : DIGIT DIGIT DIGIT DIGIT ; // year
MOY : (DIGIT | DIGIT DIGIT) ; // month of year.
DOM : (DIGIT | DIGIT DIGIT) ; // day of month.
TIMESPAN : startTime (WS THRU WS endTime)? ;
// Time-of-day.
startTime : TOD ;
endTime : TOD ;
TOD : NOON | HOUR2 (':' MINUTE)? WS? MERIDIAN ;
NOON : 'noon' ;
HOUR2 : (DIGIT | DIGIT DIGIT) ;
MERIDIAN : 'AM' | 'am' | 'PM' | 'pm' ;
// 24-hour clock. Sanity-check range in listener.
HOUR : DIGIT DIGIT ;
MINUTE : DIGIT DIGIT ;
SECOND : DIGIT DIGIT ;
// Range verb.
THRU : WS ('-'|'to') WS -> skip ;
// Weekdays.
WEEK_DAY : (SHRT_DAY | LONG_DAY) ','? WS ;
SHRT_DAY : 'Sun' | 'Mon' | 'Tue' | 'Wed' | 'Thu' | 'Fri' | 'Sat' -> skip ;
LONG_DAY : 'Sunday' | 'Monday' | 'Tuesday' | 'Wednesday' | 'Thursday' | 'Friday' | 'Saturday' -> skip ;
// Months.
SHRT_MNTH : 'Jan' | 'Feb' | 'Mar' | 'Apr' | 'May' | 'Jun' | 'Jul' | 'Aug' | 'Sep' | 'Oct' | 'Nov' | 'Dec' ;
LONG_MNTH : 'January' | 'February' | 'March' | 'April' | 'May' | 'June' | 'July' | 'August' | 'September' | 'October' | 'November' | 'December' ;
DIGIT : [0-9] ;
WS : [ \t\r\n]+ -> skip ;

Answer 1

I resolved this issue by setting up a unique production rule for each sequence of digits (of length 1, 2, 3, or 4). As well, I simplified several rules -- in effect, trying to make the production rule alternatives more straightforward. Anyway, here is the final result, which does compile:
grammar DateRange;
range : 'Every' WS longDay WS 'from' WS startDate THRU endDate
| startDate THRU endDate
| startDate
;
startDate : dateTime ; endDate : dateTime ; dateTime : utc
| shrtMdy
| yyyymmdd
| longMdy
| weekDay ','? WS longMdy
;
// Dates.
utc : yyyymmdd 'T' hour ':' minute ':' second ('-'|'+') hour ':' minute ;
yyyymmdd : year '-' moy '-' dom ;
shrtMdy : moy ('/' | '-') dom ('/' | '-') year ;
longMdy : longMonth WS dom ','? optYearAndOrTime?
| shrtMonth '.'? WS dom ','? optYearAndOrTime?
;
optYearAndOrTime : WS year ','? WS timespan
| WS year
| WS timespan
;
fragment DIGIT : [0-9] ;
ONE_DIGIT : DIGIT ;
TWO_DIGITS : DIGIT ONE_DIGIT ;
THREE_DIGITS : DIGIT TWO_DIGITS ;
FOUR_DIGITS : DIGIT THREE_DIGITS ;
year : FOUR_DIGITS ; // year
moy : ONE_DIGIT | TWO_DIGITS ; // month of year.
dom : ONE_DIGIT | TWO_DIGITS ; // day of month.
timespan : (tod THRU tod) | tod ;
// Time-of-day.
tod : noon | (hour2 (':' minute)? WS? meridian?) ;
noon : 'noon' ; hour2 : ONE_DIGIT | TWO_DIGITS ;
meridian : ('AM' | 'am' | 'PM' | 'pm' | 'a.m.' | 'p.m.') ;
// 24-hour clock. Sanity-check range in listener.
hour : TWO_DIGITS ;
minute : TWO_DIGITS ;
second : TWO_DIGITS ; // we do not use seconds.
// Range verb.
THRU : WS? ('-'|'–'|'to') WS? ;
// Weekdays.
weekDay : shrtDay | longDay ; shrtDay : 'Sun' | 'Mon' | 'Tue' | 'Wed' | 'Thu' | 'Fri' | 'Sat' ; longDay : 'Sunday' | 'Monday' | 'Tuesday' | 'Wednesday' | 'Thursday' | 'Friday' | 'Saturday' ;
// Months.
shrtMonth : 'Jan' | 'Feb' | 'Mar' | 'Apr' | 'May' | 'Jun' | 'Jul' | 'Aug' | 'Sep' | 'Oct' | 'Nov' | 'Dec' ;
longMonth : 'January' | 'February' | 'March' | 'April' | 'May' | 'June' | 'July' | 'August' | 'September' | 'October' | 'November' | 'December' ;
WS : ~[a-zA-Z0-9,.:]+ ;