YAML Plugin add regex match group (#464)

* add support for match group replace

* update doc

plugin/yaml/README.md

@@ -7,7 +7,11 @@ some basic idea of yaml config file:
  * first matched `pipe` will be used.
  * any `from` in `pipe` fits `downstream` authentication will be considered as the `pipe` matched.
  * `username_regex_match` can be used to match with regex
- * `authorized_keys`, `private_key`, `known_hosts` are `path/to/target/file`, but there are also `authorized_keys_data`, `private_key_data`, `known_hosts_data` accepting base64 inline data
+
+   * to.Username can be template of regex match groups, example: `from.username: "^password_(.*?)_regex$"` and `to.username: $1"`, will match `password_user_regex` to `user`, more sytax see <https://pkg.go.dev/regexp#Regexp.Expand>
+
+ * `authorized_keys`, `known_hosts` are array `path/to/target/file` or single string, but there are also `authorized_keys_data`, `known_hosts_data` accepting base64 inline data, file and data will be merged if both are set
+ * `private_key` is `path/to/target/file`, but there are also `private_key_data` accepting base64 inline data, file wins if both are set
  * magic placeholders in path, example usage: `/path/to/$UPSTREAM_USER/file`
     * `DOWNSTREAM_USER`: supported in `private_key`, `known_hosts`
     * `UPSTREAM_USER`: supported in `authorized_keys`, `private_key`, `known_hosts`
@@ -39,20 +43,24 @@ pipes:
     username: "user"
     ignore_hostkey: true
 - from:
-    - username: "password_.*_regex"
+    - username: "^password_(.*?)_regex$"
       username_regex_match: true
   to:
     host: host-password:2222
-    username: "user"
+    username: "$1"
     ignore_hostkey: true
 - from:
     - username: "publickey_simple"
-      authorized_keys: /path/to/publickey_simple/authorized_keys
+      authorized_keys: 
+      - /path/to/publickey_simple/authorized_keys
+      - /path/to/publickey_simple/authorized_keys2
   to:
     host: host-publickey:2222
     username: "user"
     private_key: /path/to/host-publickey/id_rsa
-    known_hosts_data: "base64_known_hosts_data"
+    known_hosts_data: 
+    - "base64_known_hosts_data"
+    - "base64_known_hosts_data2"
 - from:
     - username: ".*" # catch all    
       username_regex_match: true

plugin/yaml/yaml.go

@@ -290,8 +290,21 @@ func (p *plugin) findAndCreateUpstream(conn libplugin.ConnMetadata, password str
 		for _, from := range pipe.From {
 			matched := from.Username == user
 
+			if pipe.To.Username == "" {
+				pipe.To.Username = user
+			}
+
 			if from.UsernameRegexMatch {
-				matched, _ = regexp.MatchString(from.Username, user)
+				re, err := regexp.Compile(from.Username)
+				if err != nil {
+					return nil, err
+				}
+
+				matched = re.MatchString(user)
+
+				if matched {
+					pipe.To.Username = re.ReplaceAllString(user, pipe.To.Username)
+				}
 			}
 
 			if !matched {