more username validation

cmd/welcome/main.go

@@ -4,6 +4,7 @@ import (
 	"errors"
 	"fmt"
 	"os"
+	"regexp"
 	"strings"
 
 	"git.tilde.town/tildetown/town/invites"
@@ -43,17 +44,34 @@ func promptCode() (code string, err error) {
 }
 
 func promptUsername(townData stats.TildeData) (un string, err error) {
+	usernameRE := regexp.MustCompile(`^[a-z][-a-z0-9_]*$`)
 	unPrompt := &survey.Input{
 		Message: "desired username?",
 	}
 	err = survey.AskOne(unPrompt, &un,
 		survey.WithValidator(survey.Required),
 		survey.WithIcons(surveyIconSet),
+		survey.WithValidator(func(val interface{}) error {
+			un := val.(string)
+			if len(un) > 32 {
+				return fmt.Errorf("username '%s' is too long", un)
+			}
+
+			return nil
+		}),
+		survey.WithValidator(func(val interface{}) error {
+			un := val.(string)
+			if !usernameRE.MatchString(un) {
+				return errors.New("usernames must start with a letter and only contain letters, nubers, - or _")
+			}
+
+			return nil
+		}),
 		survey.WithValidator(func(val interface{}) error {
 			un := val.(string)
 			for _, v := range townData.Users {
 				if v.Username == un {
-					return errors.New("that username is already in use, sorry")
+					return fmt.Errorf("username '%s' is already in use", un)
 				}
 			}
 			return nil