# Grammar rules for tag searching; EBNF.

# 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
# This  is a simple version of the grammar and it allows
# rather stupid expressions, like !!TAG or ((TAG))

@@grammar :: Tags

start = expression $ ;

expression = (
           | expression and_op expression
           | expression or_op expression
           | not_op expression
           | expression_parens
           | name ) ;

expression_parens = '(' expression ')' ;

name = /[a-z][a-z0-9_]+/ ;

and_op = '&' | '&&' | 'AND' | 'and' ;

or_op = '|' | '||' | 'OR' | 'or' ;

not_op = '!' | 'NOT' | 'not' ;