For purposes of forward compatability, all lines which do not match the grammar are labeled as TAPx::Parser::Result::Unknown and are not considered parse errors.

In other words,

test    ::= status positiveInteger? description? directive? | unknown
unknown ::= {character}
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Update: lines is never referenced.

Update: Thinking about the unreferenced lines, I thought of a possible simplification: Requiring one and only one (no more, no less) "plan" line should not be enforced using the grammar. In other words, treat the count of plan lines as a semantic check, not a syntactic check. This leads to a slightly simpler grammar that will be easier to maintain, especially as things are added.

tap         ::= lines
lines       ::= line {lines}
line        ::= (comment | test | plan | unknown ) "\n"
plan        ::= '1..' nonNegativeInteger "\n"
test        ::= status positiveInteger? description? directive?
status      ::= 'not '? 'ok '
description ::= (character - (digit '#')) {character - '#'}
directive   ::= '#' ( 'TODO' | 'SKIP' ) ' ' {character}
comment     ::= '#' {character}
unknown     ::= {character}

digit              ::= [:digit:]
character          ::= ([:print:] - "\n")
positiveInteger    ::= ( digit - '0' ) {digit}
nonNegativeInteger ::= digit {digit}
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This doesn't mean it can't be checked at parse-time. Make sure a plan wasn't already specified when one is encountered. Make sure a plan was specified when parsing is done.