BNF grammar should be whitespace-insensitive for epsilon productions

Problem

The hand-coded BNF grammar in lib/Chalk/Grammar/BNF.pm is currently sensitive to trailing whitespace in epsilon productions. This causes parse failures for valid BNF syntax.

Current Behavior

Works: Rule ->\n (no trailing whitespace) Fails: Rule -> \n (trailing space before newline)

Root Cause

The GrammarRule pattern uses qr/\s*/ after ->, which matches ALL whitespace including newlines:

['GrammarRule', [
    qr/[A-Z][a-zA-Z0-9_]*/,  # LHS
    qr/\s*/,                  # Optional whitespace
    '->',
    qr/\s*/,                  # ← This consumes the newline!
    'RHS'
]],

When trailing whitespace exists, the qr/\s*/ matches both the space AND the newline, leaving nothing for the Line rule to match:

['Line', ['GrammarRule', "\n"]],  # ← No newline left to match

Impact

• Phase 7 Workaround: Removed trailing whitespace from 4 epsilon productions in grammar/perl.bnf (lines 243, 288, 371, 462)
• Future Risk: Any BNF file with trailing whitespace will fail to parse

Attempted Solution

Changing qr/\s*/ to qr/[ \t]*/ (horizontal whitespace only) was attempted but caused other parsing failures. The issue is more complex than a simple pattern change.

Proposed Solutions

Option 1: Restructure GrammarRule pattern

• Use negative lookahead to prevent consuming newlines: qr/(?:[^\S\n]*)/
• May require adjusting other patterns that expect RHS to consume trailing whitespace

Option 2: Preprocess BNF content

• Strip trailing whitespace before parsing
• Simple but doesn't address root cause

Option 3: Restructure grammar rules

• Separate epsilon productions from normal productions
• GrammarRule -> LHS '->' RHS | LHS '->' EpsilonMarker
• More explicit but requires grammar redesign

Recommendation

Start with Option 1 (pattern fix) with comprehensive testing. The core issue is that qr/\s*/ is too greedy at the end of GrammarRule. We need a pattern that:

1. Matches horizontal whitespace after ->
2. Does NOT consume newlines
3. Works correctly for both epsilon and non-epsilon productions

Test Case

# Should parse successfully with semantic actions
my $bnf = <<'BNF';
Rule1 -> 
Rule2 -> Element
Rule3 -> Element1 Element2
BNF

my $g = Chalk::BNF::parse_with_semantic_actions($bnf);
# Currently fails if Rule1 has trailing space

Related Work

• Phase 7 completion: #63
• Semantic actions architecture: Feature branch feature/semantic-actions-architecture
• Working epsilon productions: Lines 197, 285, 371, 462 in perl.bnf (no trailing whitespace)