# Grammar rules for tag searching; BNF.

# 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 - & &&
#                    disjunction - | ||
#                    negation    - !
# This  is a simple version of the grammar and it allows
# rather stupid expressions, like !!TAG or ((TAG)); in the future
# it will be fixed by making the grammar more complex and stricter.

NAME : '[a-z][a-z0-9_]+'

AND_OP : '&'

OR_OP : '|'

NOT_OP : '!'

SP1 : '[ \t]+'

expression : NAME
           | expression AND_OP AND_OP expression
           | expression AND_OP expression
           | expression OR_OP OR_OP expression
           | expression OR_OP expression
           | NOT_OP expression
           | '(' expression ')'