The ex command <code>g</code> is very useful for acting on lines that match a pattern. You can use it with the <code>d</code> command, to delete all lines that contain a particular pattern, or all lines that do not contain a pattern.

For example, to delete all lines containing "profile" (the first command is optional; it shows the lines that the second command will delete):
<pre>
:g/profile
:g/profile/d
</pre>

More complex patterns can be used, such as deleting all lines that are empty or that contain only whitespace:
<pre>
:g/^\s*$/d
</pre>

To delete all lines that do ''not'' contain a pattern, use <code>g!</code>, like this command to delete all lines that are not comment lines in a Vim script:
<pre>
:g!/^\s*"/d
</pre>

Note that <code>g!</code> is equivalent to <code>v</code>, so you could also do the above with:
<pre>
:v/^\s*"/d
</pre>

The next example shows use of <code>\|</code> ("or") to delete all lines ''except'' those that contain "<code>error</code>" or "<code>warn</code>" or "<code>fail</code>" ({{help|pattern}}):
<pre>
:v/error\|warn\|fail/d
</pre>