When _ra_comp() in _regex_arguments() gets called ?

[Han Pingtian <hanpt@xxxxxxxxxxxxxxxxxx>]

Hey,

I'm trying to understand the utility function "_regex_arguments()" and
stuck at when the _ra_comp() is called. This function is placed in array
"regex" something like ":_ra_comp :'compadd aaa'". There is a colon
before its name. What does the colon mean here?

Thanks!