# Grammar rules for tag searching

# The grammar defines expressions in the following forms:
#  TAG - search blog posts that contain the tag;
#  !TAG - search blog posts that don't contain the tag;
#  TAG & TAG - search blog posts that contain both tags;
#  TAG | TAG - search blog posts that contain any of the tags;
# Parentheses are allowed to group expressions; for example:
#  TAG & (TAG | TAG)
#  !(TAG | TAG)
# Allowed operators: conjunction - & && AND and
#                    disjunction - | || OR or
#                    negation    - ! NOT not
# Usual priority: NOT recognized before AND, AND before OR.
# This  is a simple version of the grammar and it allows
# rather stupid expressions, like (TAG) or ((TAG)) or !(!(TAG)).

expression = or_expression / aterm_expression

or_expression = aterm_expression or_op expression

and_expression = term_expression and_op aterm_expression

not_expression = not_op space0 (parens_expression / name)

aterm_expression = and_expression / term_expression

term_expression = not_expression / parens_expression / (name space_b4letter)

parens_expression = "(" space0 expression space0 ")"

and_op = (space0 ("&&" / "&") space0) / (space0 ("AND" / "and") space_b4letter)

or_op = (space0 ("||" / "|") space0) / (space0 ("OR" / "or") space_b4letter)

not_op = (space0 "!" space0) / (space0 ("NOT" / "not") space_b4letter)

letter = ~"[a-z]"i

name = ~"[a-z][a-z0-9_]*"

space_b4letter = (space1 &letter) / space0

space0 = ~" *"

space1 = ~" +"

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