Anchors ^ and $

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.

^^ or \A The beginning of a file
$$ or \Z 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
\b Word boundary (beginning or end)
\B Not word boundary (between two letters or two spaces)

For example:

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

This is a page from Aba Search and Replace help file.