There are several sections in the docs where we talk about the supported regex options. In fact, we only support 4: g ,i ,m, and s. Please update the BSON Spec to reflect this. 
See attached ticket's comments for details as needed.
The bottom line is that the EJSON v2 page is accurate.

Documentation changes:

Right now, the site says:

Options are identified by characters, which must be stored in alphabetical order. Valid options are 'i' for case insensitive matching, 'm' for multiline matching, 'x' for verbose mode, 'l' to make \w, \W, etc. locale dependent, 's' for dotall mode ('.' matches everything), and 'u' to make \w, \W, etc. match unicode. 

But only i,  m, s, x, and u, or an empty string "" are supported currently.

• i: Case-insensitive search
• m: Allows ^ and $ to match next to newline characters
• s: Allows . to match newline characters
• x: Free-spacing and line comments
• u: Unicode
• "":