If you are familiar with regular expressions, I would say it is
^(?!\d)\w{1,40}$
Where \w
refers to the range of Unicode word characters, which also contain the underscore and the digits from 0-9.
In plain English: The bookmark name must...
- be between 1 and 40 characters long
- consist of any combination of Unicode letters, digits, underscores
- not start with a digit
- not contain any kind of white space or punctuation
As stated in the comments, bookmark names beginning with an underscore are treated as hidden. They will not appear in the regular user interface, but they can be used from VBA code. It it is not possible to create bookmarks that begin with an underscore via the regular user interface, but you can do it through VBA code with Bookmarks.Add()
.