How do I duplicate every line matching a pattern, placing a copy directly below each one?

Explanation

Combining :global with the :t (copy) command and the . address creates a powerful one-liner: for every line matching a pattern, a copy is inserted immediately below it. Unlike :g/pattern/t$ which moves all matches to the end of the file, t. keeps each copy in context right next to the original.

How it works

• :g/pattern marks every line that matches the pattern
• t. means "copy the current line to after line ." — and . here refers to the current matched line itself
• So each matched line is duplicated directly below itself
• Vim marks all lines before any edits begin, so newly inserted copies are never re-processed

Example

function foo() {}
let x = 1;
function bar() {}

After :g/^function/t.:

function foo() {}
function foo() {}
let x = 1;
function bar() {}
function bar() {}

Tips

• To place the copy above the original instead, use t.-1 (insert before the current line)
• Useful as a "safety copy" before bulk edits: duplicate matching lines, then modify the originals while keeping the untouched copies nearby
• Combine with a substitute on the next line: :g/^TODO/t. | +s/^TODO/DONE/ duplicates each TODO line then marks the original as DONE