Insert line above matched line

When used in a substitution command, & is replaced by the text that matches the search pattern. If we match a pattern from the beginning of a line, we can insert something above it by replacing the match with the new content, a line break and the matched pattern.

export class MyClass {
  property: string;
}

:%s/^export class/\/\/ This is a class\r&/

+ // This is a class
  export class MyClass {
    property: string
  }

Source: :h &