[jQuery] Help with getting variable

[Dave Maharaj :: WidePixels.com]

Hoping for some simple help here.

I have this structure in my forms 

name="data[User][username]"

But depending on the form it the ['User'] section will change throught the
site. How can I pull the User as a variable?

The script has something like this:

$.post('/ajax_validate/users/', {
                                        field: fieldName,
                                        value: fieldValue
                                        },

Where I want to add the variable like

$.post('/ajax_validate/'+myVarHere, {
                                        field: fieldName,
                                        value: fieldValue
                                        },
 
Thanks,

Dave