Conditional subpatterns
Recursive patterns
PCRE regex syntax
PHP Manual

## Comments

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.

Conditional subpatterns
Recursive patterns
PCRE regex syntax
PHP Manual