Quoting and a bit paraphrasing "Mastering Regular Expressions 2nd Edition":
A match mode can change the meaning of "$" to match before any embedde
+d newline (or Unicode line terminator as well). When supported, "\Z"
+usually matches what the "unmoded" "$" matches, which often means to
+match at the end of the string, or before a string-ending newline. To
+ complement these, "\z" matches only at the end of the string, period
+, without regard to any newline.
//s stands for Single Line Mode which makes the dot match any characte
//m stands for Multi Line Mode which changes how ^& $ are considered b
+y the regex engine. ^ is then begin of 1 line out of the many lines i
+n the string and not begin of string and $ is end of 1 line out of th
+e many lines in the string and not end of string.
Caret "^" matches at the beginning of the text being searched, and, if
+ in an enhanced line-anchor match mode after any newline.
\A always matches only at the start of the text being searched, regard
+less of single or multi line match mode.
"\Z" matches what the "unmoded" "$" matches, which means to match at t
+he end of the string, or before a string-ending newline. To complemen
+t these, "\z" matches only at the end of the string, period, without
+regard to any newline.
With thanks to Jeffrey Friedl's Regex Holy Book! ;-)