Warning: preg_match(): Delimiter must not be alphanumeric, backslash, or NUL in admin\controller\customer\custom_field.php on line 387{"error":{"validation":"error_validation"}}

Also I don't see an warning message in admin like...This regex is invalid...

No error_validation in language file

Warning: preg_match(): Delimiter must not be alphanumeric, backslash, or NUL in admin\controller\customer\custom_field.php on line 387{"error":{"validation":"error_validation"}}

That's a separate issue and you can avoid it by entering a valid regex pattern as the validation rule.

yes...but there is no warning message to display in admin.....

Ok, but it's not going to be something I solve in this PR. I would suggest you create another issue about it.