Zero Width Perl Escape Codes

Zero width Perl Escape sequences	Meaning
\A	Match only at beginning of string
\b	Match a word boundary, which is a spot between two characters that has a "\b" one one side and a "\w" on the other side in either order, counting the imaginary characters off the beginning or end of the string as a match for "\W". Within character classes "\b" represents backspace rather than a word boundary.
\B	Match a non-word boundary
\G	Match only at pos(), i.e. at the end-of-match position of prior m//g
\z	Match only at end of string. Use to match actual end of string and not ignore an optional trailing newline.
\Z	Match only at end of string, or before newline at the end.

Note: "\A" is same as "^", and "\Z" is same as "$", except that the former
("\A" and "\Z") won't match mutiple times when the "/m" modifier is used,
while "^" and "$" match at every internal line boundary.

Zero width perl lookahead and lookbehind assertions (generalizations of the anchor concept):

look ahead assertion: "(?=regexp)"
negated look ahead assertion: "(?!regexp)"
look behind assertion: "(?<=fixed_width_regexp)"
negated look behind assertion: "(?<!fixed_width_regexp)"

The negated versions of the assertion evaluate true if the regexps do not match.
Note that these are non capturing into the $n variables because they are zero width.