# 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 one of the tags or both;
-# Parentheses are allowed to group expressions:
+# TAG | TAG - search blog posts that contain any of the tags;
+# Parentheses are allowed to group expressions; for example:
# TAG & (TAG | TAG)
# !(TAG | TAG)
-# and so on. This is the first version of the grammar and it allows
+# Allowed operators: conjunction - & && AND and
+# disjunction - | || OR or
+# 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 stricter and more complex.
+# it will be fixed by making the grammar more complex and stricter.
NAME : '[a-z][a-z0-9_]+'
NOT_OP : '!'
-SP0 : '[ \t]*'
-
SP1 : '[ \t]+'
expression : NAME
- | expression AND_OP expression
- | expression OR_OP expression
- | NOT_OP expression
- | '(' expression ')'
- | SP0 expression SP0
+ | expression SP0 AND_OP AND_OP SP0 expression
+ | expression SP0 AND_OP SP0 expression
+ | l_expression and_word r_expression
+ | NOT_OP SP0 expression
+ | expression SP0 OR_OP OR_OP SP0 expression
+ | expression SP0 OR_OP SP0 expression
+ | l_expression or_word r_expression
+ | expression_parens
+
+l_expression : expression_parens
+ | expression_sp
+
+r_expression : expression_parens
+ | sp_expression
+
+expression_parens : '(' SP0 expression SP0 ')'
+
+sp_expression : SP1 expression
+
+expression_sp : expression SP1
+
+and_word : 'A' 'N' 'D'
+ | 'a' 'n' 'd'
+
+or_word : 'O' 'R'
+ | 'o' 'r'
+
+SP0 : SP1 | empty
+
+empty :