Regular expression for a string that does not start with a sequence

Question

You could use a negative look-ahead assertion:

^(?!tbd_).+

Or a negative look-behind assertion:

(^.{1,3}$|^.{4}(?<!tbd_).*)

Or just plain old character sets and alternations:

^([^t]|t($|[^b]|b($|[^d]|d($|[^_])))).*