4.5 Zero Address

As a GNU sed extension, 0 address can be used in these cases:

1. In a regex range address as 0,/regexp/ (see Zero Address Regex Range).
2. With the r command, inserting a file before the first line (see Adding a header to multiple files).
3. As the starting address in the first~step extension (see Selecting lines by numbers).

Note that these are the only places where the 0 address makes sense; Commands which are given the 0 address in any other way will give an error.

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