How do I copy the contents of one register into another register?

Explanation

Vim's :let command lets you read and write register contents as strings, making it possible to copy, combine, or modify register values without ever leaving the editor. Copying the unnamed register @" into a named register before it gets overwritten is a practical pattern for preserving yanked text during complex multi-step edits.

How it works

• :let assigns an expression to a variable or register
• @a on the left side refers to the named register a as an lvalue
• @" on the right side reads the current contents of the unnamed register
• This copies the unnamed register's text into register a, leaving both intact

You can copy between any two registers:

:let @b = @a    " copy register a into b
:let @" = @a   " copy named register a back into the unnamed register
:let @+ = @"   " push unnamed register to the system clipboard

Example

You yank a line with yy, but then need to delete several lines (which would overwrite @"):

Before: @" contains "important text\n"

:let @a = @"

After: @a contains "important text\n", safe from future deletes

Now paste with "ap at any point, even after many deletions.

Tips

• Extend this to transform register contents: :let @a = substitute(@a, 'foo', 'bar', 'g') runs a substitution on the register value itself
• :let @a = toupper(@a) uppercases everything in the register without a buffer roundtrip
• :reg a inspects the current value of register a before and after