Beginning of Pattern Space Anchor

The ^ anchor generally means “match the null position at the beginning of a line”, although you can generally think like that in Sed as well, sometimes it can lead to wrong assumptions.

The input file contains:

foo
bar
jedi

If you do

sed '=' file.txt

the output is

1
foo
2
bar
3
jedi

Now you do:

sed '=' | sed 'N; s/^/      /' file.txt

and the output, suprisingly, is:

    1
foo
    2
bar
    3
jedi

As you see, only the odd output lines were indented with 6 spaces, the even ones were left untouched, even though we used `s/^/ /’. Why isn’t Sed adding spaces in the beginning of all lines? Read further along to discover.

When we do N, we add a newline to the pattern space, and then the next line of input. That means that sed '=' | sed 'N' causes the pattern space to hold

1\nfoo

then

2\nbar

then

3\njedi

We think, for instance, that “2” is one line, and “bar” is another line, and they actually are each one a separate line. The pattern space even contains a \n between them, and that means the pattern space really contains two lines.

Here comes the gotcha: in Sed, ^ means “match the null position at the beginning of the pattern space” (and not at the beginning of a line).

That means that even though when the pattern space contains 3\njedi (and that means we have two lines in the pattern space), s/^/ / will only add 6 spaces in front of “3”, leaving the “jedi” part alone. It will add 6 spaces at the beginning of the pattern space, and not at the beginning of the line (or lines).

Remember this: When it comes to regex, ^ generally means “the null position at the beginning of a line”, but in Sed it really means “the null position at the beginning of the pattern space.”

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