Regular expression
A regular expression is a pattern that is matched against a subject string from left to right. Most characters stand for themselves in a pattern, and match the corresponding characters in the subject.
The syntax for patterns used in these functions closely resembles Perl.
Delimiters can be any non-alphanumeric, non-whitespace ASCII character except the backslash (\) and the null byte. If the delimiter character has to be used in the expression itself, it needs to be escaped by backslash.
The PCRE (Perl Compatible Regular Expressions) library is a set of functions that implement regular expression pattern matching using the same syntax and semantics as Perl 5, with just a few differences.
Regular expressions using a POSIX-extended syntax were DEPRECATED in PHP 5.3.0, and REMOVED in PHP 7.0.0.
preg_match($pattern, $subject, $matches);
preg_match_all($pattern, $subject, $matches);
preg_grep($pattern, $input);
preg_split($pattern, $subject, $limit, $flags);
preg_filter($pattern, $replacement, $subject);
preg_replace_callback($pattern, $callback, $subject);
preg_replace_callback_array($patterns_and_callbacks, $subject);
preg_last_error();
preg_quote($str, $delimiter);