Question

What are the differences between,

\A  Match at only beginning of string
\Z  Match at only end of string (or before newline at the end)

^   Match the beginning of the line
$   Match the end of the line (or before newline at the end)

Answer 1

From the perl documentation (I can’t see them in the standard regex syntax):

The “\A” and “\Z” are just like “^” and “$”, except that they won’t match multiple times when the “/m” modifier is used, while “^” and “$” will match at every internal line boundary.

You should be adding a perl tag if this is related just to Perl (or those languages or libraries using PCRE, Perl-Compatible Regular Expressions).