Revert "MM-54502 - Update regex to force first character to be alpha … (#28860)

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

This reverts commit 08ed72f060.

* update tests for reverted PR

* Revert "update tests for reverted PR"

This reverts commit c1cf5495fa129008e44a8fcdc0ee34762b33dd34.

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

This reverts commit 6920b979c8dba86a837bafff7c057dc609c6aa58.

* only revert important parts, not tests

---------

Co-authored-by: Mattermost Build <build@mattermost.com>

server/public/model/user.go

@@ -976,8 +976,7 @@ func HashPassword(password string) (string, error) {
 }
 
 var validUsernameChars = regexp.MustCompile(`^[a-z0-9\.\-_]+$`)
-var validUsername = regexp.MustCompile(`^[a-z][a-z0-9\.\-_]*$`)
-var validUsernameCharsForRemote = regexp.MustCompile(`^[a-z][a-z0-9\.\-_:]*$`)
+var validUsernameCharsForRemote = regexp.MustCompile(`^[a-z0-9\.\-_:]*$`)
 
 var restrictedUsernames = map[string]struct{}{
 	"all":       {},
@@ -991,7 +990,7 @@ func IsValidUsername(s string) bool {
 		return false
 	}
 
-	if !validUsername.MatchString(s) {
+	if !validUsernameChars.MatchString(s) {
 		return false
 	}
 

server/public/model/user_test.go

@@ -431,9 +431,9 @@ var usernames = []usernamesTest{
 	{"spin-punch", true, true},
 	{"sp", true, true},
 	{"s", true, true},
-	{"1spin-punch", false, false},
-	{"-spin-punch", false, false},
-	{".spin-punch", false, false},
+	{"1spin-punch", true, true},
+	{"-spin-punch", true, true},
+	{".spin-punch", true, true},
 	{"Spin-punch", false, false},
 	{"spin punch-", false, false},
 	{"spin_punch", true, true},