In regular expressions the boundaries ^ and \A can only match at the beginning of the input (or, in case of
^ in combination with the MULTILINE flag, the beginning of the line) and $, \Z and \z
only at the end.
These patterns can be misused, by accidentally switching ^ and $ for example, to create a pattern that can never
match.
Noncompliant code example
// This can never match because $ and ^ have been switched around
preg_match("/$[a-z]+^/", $str); // Noncompliant
Compliant solution
preg_match("/^[a-z]+$/", $str);
