Memory App

Start, End (of the line)

Find /cat/ at the beginning of the line.

Find /cat/ at the end of the line.


Interpretation in a literal way

It's best to get into the habit of interpreting regular expressions in a rather literal way. Don't think ^cat matches if you have:

a line with cat at the beginning
But rather ^cat mathces if you have:

the beginning of a line, 
followed immediately by c, 
followed immediately by a,
followed immediately by t (2/3)

Empty line

Match empty line:

Matches if the line has a beginning-of-line, followed immediately by the end of the line. Effectively means: an empty line (with nothing on it, not even spaces).