Adding upstream version 2.5.1.

Signed-off-by: Daniel Baumann <daniel@debian.org>

index/scorch/regexp.go

@@ -0,0 +1,63 @@
+//  Copyright (c) 2020 Couchbase, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// 		http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package scorch
+
+import (
+	"regexp/syntax"
+
+	"github.com/blevesearch/vellum/regexp"
+)
+
+func parseRegexp(pattern string) (a *regexp.Regexp, prefixBeg, prefixEnd []byte, err error) {
+	// TODO: potential optimization where syntax.Regexp supports a Simplify() API?
+
+	parsed, err := syntax.Parse(pattern, syntax.Perl)
+	if err != nil {
+		return nil, nil, nil, err
+	}
+
+	re, err := regexp.NewParsedWithLimit(pattern, parsed, regexp.DefaultLimit)
+	if err != nil {
+		return nil, nil, nil, err
+	}
+
+	prefix := literalPrefix(parsed)
+	if prefix != "" {
+		prefixBeg := []byte(prefix)
+		prefixEnd := calculateExclusiveEndFromPrefix(prefixBeg)
+		return re, prefixBeg, prefixEnd, nil
+	}
+
+	return re, nil, nil, nil
+}
+
+// Returns the literal prefix given the parse tree for a regexp
+func literalPrefix(s *syntax.Regexp) string {
+	// traverse the left-most branch in the parse tree as long as the
+	// node represents a concatenation
+	for s != nil && s.Op == syntax.OpConcat {
+		if len(s.Sub) < 1 {
+			return ""
+		}
+
+		s = s.Sub[0]
+	}
+
+	if s.Op == syntax.OpLiteral && (s.Flags&syntax.FoldCase == 0) {
+		return string(s.Rune)
+	}
+
+	return "" // no literal prefix
+}