The sequence (?# marks the start of a comment which continues up to the next closing parenthesis. Nested parentheses are not permitted. The characters that make up a comment play no part in the pattern matching at all.
If the PCRE_EXTENDED option is set, an unescaped # character outside a character class introduces a comment that continues up to the next newline character in the pattern.
Example #1 Usage of comments in PCRE pattern
<?php
$subject = 'test';
/* (?# can be used to add comments without enabling PCRE_EXTENDED */
$match = preg_match('/te(?# this is a comment)st/', $subject);
var_dump($match);
/* Whitespace and # is treated as part of the pattern unless PCRE_EXTENDED is enabled */
$match = preg_match('/te #~~~~
st/', $subject);
var_dump($match);
/* When PCRE_EXTENDED is enabled, all whitespace data characters and anything
that follows an unescaped # on the same line is ignored */
$match = preg_match('/te #~~~~
st/x', $subject);
var_dump($match);
The above example will output:
int(1) int(0) int(1)