Avoid word-splitting regular expression matches

Previously, we would conduct regular expression searches by
enumerating all values of a given term, manually regex-matching each
one against our search regular expression, remember all the term
values that matched our regular expression, then do a big Xapian
OR-query that matched any of those term values. In constructing this
OR-query, however, we would split each term value on space and add a
separate Xapian phrase search term for each resulting word. This
approach worked fine most of the time, beacuse when we index a term,
we index both each word in a term and the whole term by itself.

This word splitting produced false negatives in some matches, however,
because Xapian and the Mu-level word splitting code do word splitting
slightly differently and apply different transformations to the text
while splitting.  (For example, Xapian transforms fancy Unicode
apostrophes to ASCII apostrophes.)

This patch avoids the problem by not word splitting when constructing
the big Xapian OR-query for finding the results of regular
expression matching.

lib/mu-parser.cc

@@ -267,7 +267,7 @@ Parser::Private::regex(const FieldInfoVec& fields,
 		for (const auto& field : fields) {
 			const auto terms = process_regex(field.field, rx);
 			for (const auto& term : terms) {
-				tree.add_child(Tree({Node::Type::Value,
+				tree.add_child(Tree({Node::Type::ValueAtomic,
 							FieldValue{field.id, term}}));
 			}
 		}

lib/mu-tree.hh

@@ -80,6 +80,7 @@ struct Node {
 		OpAndNot,
 		OpNot,
 		Value,
+                ValueAtomic,
 		Range,
 		Invalid
 	};
@@ -107,6 +108,8 @@ struct Node {
 			return "not";
 		case Type::Value:
 			return "value";
+		case Type::ValueAtomic:
+			return "value_atomic";
 		case Type::Range:
 			return "range";
 		case Type::Invalid:

lib/mu-xapian.cc

@@ -83,11 +83,15 @@ xapian_query_value(const Mu::Tree& tree)
 		return make_query(field_val, true /*maybe-wildcard*/);
 	}
 
+        const bool is_atomic = tree.node.type == Node::Type::ValueAtomic;
+
 	const auto parts{split(field_val.value(), " ")};
 	if (parts.empty())
 		return Xapian::Query::MatchNothing; // shouldn't happen
-	else if (parts.size() == 1)
+	else if (parts.size() == 1 && !is_atomic)
 		return make_query(field_val, true /*maybe-wildcard*/);
+        else if (is_atomic)
+                return make_query(field_val, false /*maybe-wildcard*/);
 
 	std::vector<Xapian::Query> phvec;
 	for (const auto& p : parts) {
@@ -124,6 +128,7 @@ Mu::xapian_query(const Mu::Tree& tree)
 	case Node::Type::OpAndNot:
 		return xapian_query_op(tree);
 	case Node::Type::Value:
+	case Node::Type::ValueAtomic:
 		return xapian_query_value(tree);
 	case Node::Type::Range:
 		return xapian_query_range(tree);