What does ? mean in a directive's require property

Question

Some directives use some cryptic chars in their require property, like the angular ui dropdow

require:"?^dropdown"

I'm ok with ^ which mean parent but I can't figured out what ? means.

Thank you!

Answer 1

As the code says:

[
    'siblingDirectiveName', 
    '^parentDirectiveName', 
    '?optionalSiblingDirectiveName', 
    '?^optionalParentDirectiveName'
]

Answer 2

check the docs: $compile 'require'

the directive docs have been moved there a while ago

have a nice weekend!