where to look for the variable
the variable's name
regular expression to filter which variables we're interested in. Use null to indicate all
the function to be called on each value of the variable
opaque pointer to pass to the callback
0 or an error code.
Get each value of a multivar in a foreach callback
The callback will be called on each variable found
The regular expression is applied case-sensitively on the normalized form of the variable name: the section and variable parts are lower-cased. The subsection is left unchanged.