Match Start and End of Line

^abc and \Aabc match abc at the start of whole string
abc$ and abc\Z match abc at the start of whole string
^abc match abc at the start of each line
abc$ match abc at the start of each line

cs

_ = new Regex(@"^abcefg$", RegexOptions.Multiline);

^ always matches after \n, so \n^ is redundant
$ always matches before \n, so $\n is redundant
\A\Z matches empty string and empty string with a single new line
\A\z matches only empty string

TIP

Always use \A and \Z instead of ^ and $ when to match start/end of a whole string

Mode modifier

Use (?m)/(?-m) to enable/disable multiline mode in regex literal

Conclusion

\A and \Z always match the start and end of a subject string
(?-m)^abc and (?-m)$abc are equivalent to \Aabc and \Zabc
\z matches the end of the subject string
abc\Z matches before line break while abc\z won't match if line break exists after abc