Anchors match at the beginning of a file, a line, or a word. All of them are “zero-width”, that is, they don't match any characters in the source file.

^^	The beginning of a file
$$	The end of a file
^	The beginning of a line
$	The end of a line
\<	The beginning of a word
\>	The end of a word

For example:

^From	A line starting with “From”
ing\>	A word ending with “ing”
^abc$	A line containing only “abc”
^#include	A line starting with “#include”
^$	An empty line
^^$$	An empty file