MM-54502 - Update regex to force first character to be alpha (#24675)

Automatic Merge

server/public/model/user.go

@@ -453,7 +453,7 @@ func (u *User) PreSave() *AppError {
 	}
 
 	if u.Username == "" {
-		u.Username = NewId()
+		u.Username = NewUsername()
 	}
 
 	if u.AuthData != nil && *u.AuthData == "" {
@@ -972,7 +972,8 @@ func HashPassword(password string) (string, error) {
 }
 
 var validUsernameChars = regexp.MustCompile(`^[a-z0-9\.\-_]+$`)
-var validUsernameCharsForRemote = regexp.MustCompile(`^[a-z0-9\.\-_:]+$`)
+var validUsername = regexp.MustCompile(`^[a-z][a-z0-9\.\-_]*$`)
+var validUsernameCharsForRemote = regexp.MustCompile(`^[a-z][a-z0-9\.\-_:]*$`)
 
 var restrictedUsernames = map[string]struct{}{
 	"all":       {},
@@ -986,7 +987,7 @@ func IsValidUsername(s string) bool {
 		return false
 	}
 
-	if !validUsernameChars.MatchString(s) {
+	if !validUsername.MatchString(s) {
 		return false
 	}
 
@@ -1028,7 +1029,7 @@ func CleanUsername(logger mlog.LoggerIFace, username string) string {
 	s = strings.Trim(s, "-")
 
 	if !IsValidUsername(s) {
-		s = "a" + NewId()
+		s = NewUsername()
 		logger.Warn("Generating new username since provided username was invalid",
 			mlog.String("provided_username", username), mlog.String("new_username", s))
 	}