The syntax for patterns used in these functions closely resembles Perl. The expression should be enclosed in the delimiters, a forward slash (/), for example. Any character can be used for delimiter as long as it’s not alphanumeric or backslash (\). If the delimiter character has to be used in the expression itself, it needs to be escaped by backslash. Since PHP 4.0.4, you can also use Perl-style (), {}, [], and <> matching delimiters. See Pattern Syntax for detailed explanation.

摘自php manual

任何非字母，数字和\的字符都可以当做delimiter，好处，估计是可以不和expression里面的字符冲突，比如，如果里面有一个/，那么就用#好了。

知识的力量。