<?php
$string = 'test';
echo preg_match('/te(?# comments)st/', $string) . "\n";
echo preg_match('/te#~~~~
st/', $string) . "\n";
echo preg_match('/te#~~~~
st/x', $string) . "\n";
// result
// 1
// 0
// 1
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.
<?php
$string = 'test';
echo preg_match('/te(?# comments)st/', $string) . "\n";
echo preg_match('/te#~~~~
st/', $string) . "\n";
echo preg_match('/te#~~~~
st/x', $string) . "\n";
// result
// 1
// 0
// 1