Anchors

Outside a character class, in the default matching
mode, the circumflex character (^) is an assertion which
is true only if the current matching point is at the start of the
subject string. Inside a character class, circumflex (^)
has an entirely different meaning (see below).

Circumflex (^) need not be the first
character of the pattern if a number of alternatives are involved,
but it should be the first thing in each alternative in which it
appears if the pattern is ever to match that branch. If all
possible alternatives start with a circumflex (^), that
is, if the pattern is constrained to match only at the start of the
subject, it is said to be an “anchored” pattern. (There are also
other constructs that can cause a pattern to be anchored.)

A dollar character ($) is an assertion
which is TRUE only if the current
matching point is at the end of the subject string, or immediately
before a newline character that is the last character in the string
(by default). Dollar ($) need not be the last character of
the pattern if a number of alternatives are involved, but it should
be the last item in any branch in which it appears. Dollar has no
special meaning in a character class.

The meaning of dollar can be changed so that it
matches only at the very end of the string, by setting the PCRE_DOLLAR_ENDONLY option at compile or matching time.
This does not affect the \Z assertion.

The meanings of the circumflex and dollar
characters are changed if the PCRE_MULTILINE option is set. When this is the case,
they match immediately after and immediately before an internal
“\n” character, respectively, in addition to matching at the start
and end of the subject string. For example, the pattern /^abc$/
matches the subject string “def\nabc” in multiline mode, but not
otherwise. Consequently, patterns that are anchored in single line
mode because all branches start with “^” are not anchored in
multiline mode. The PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.

Note that the sequences \A, \Z, and \z can be used
to match the start and end of the subject in both modes, and if all
branches of a pattern start with \A is it always anchored, whether
PCRE_MULTILINE is set or not.