[#672] Fix handling X-Amz-Copy-Source header

Signed-off-by: Denis Kirillov <denis@nspcc.ru>

api/auth/regexp_utils.go

@@ -2,11 +2,17 @@ package auth
 
 import "regexp"
 
-type regexpSubmatcher struct {
+type RegexpSubmatcher struct {
 	re *regexp.Regexp
 }
 
-func (r *regexpSubmatcher) getSubmatches(target string) map[string]string {
+// NewRegexpMatcher creates a new regexp sub matcher.
+func NewRegexpMatcher(re *regexp.Regexp) *RegexpSubmatcher {
+	return &RegexpSubmatcher{re: re}
+}
+
+// GetSubmatches returns matches from provided string. Zero length indicates no match.
+func (r *RegexpSubmatcher) GetSubmatches(target string) map[string]string {
 	matches := r.re.FindStringSubmatch(target)
 	l := len(matches)