How to search and replace each hit with a multiline register in which each hit is replaced with an individual line from the register?

Question

so if i have the following multiline text in my register +:

ZERO     
SIGN     
CARRY    
AUXILIARY
PARITY   
OVERFLOW 
INTERRUPT
DIRECTION
TRAP     

And I have the following code:

case ZERO     :
  return a->FLAGS.derp;
case SIGN     :
  return a->FLAGS.derp;
case CARRY    :
  return a->FLAGS.derp;
case AUXILIARY:
  return a->FLAGS.derp;
case PARITY   :
  return a->FLAGS.derp;
case OVERFLOW :
  return a->FLAGS.derp;
case INTERRUPT:
  return a->FLAGS.derp;
case DIRECTION:
  return a->FLAGS.derp;
case TRAP     :
  return a->FLAGS.derp;

I want the result of a search and replace command (i.e. :%s\derp\register+\g) to be

case ZERO     :
  return a->FLAGS.ZERO;
case SIGN     :
  return a->FLAGS.SIGN;
case CARRY    :
  return a->FLAGS.CARRY;
case AUXILIARY:
  return a->FLAGS.AUXILIARY;
case PARITY   :
  return a->FLAGS.PARITY;
case OVERFLOW :
  return a->FLAGS.OVERFLOW;
case INTERRUPT:
  return a->FLAGS.INTERRUPT;
case DIRECTION:
  return a->FLAGS.DIRECTION;
case TRAP     :
  return a->FLAGS.TRAP;

The command :%s/foo/\=@a/g is very close to this but not close enough, it will insert the totality of the register on each match.

Answer 1

One way to do this is using :h sub-replace-expression with the list flag of getreg(), assuming your code starts at line 1:

%s;derp;\=getreg('+',1,1)[float2nr((line('.')-1)/2)]

If your register looked like this (e.g., using a scratch buffer to duplicate lines):

ZERO     
ZERO     
SIGN     
SIGN     
CARRY  
CARRY   
&c...

this would simplify to

%s;derp;\=getreg('+',1,1)[line('.')]

which is a handy pattern in many situations.