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Nothing really. PHP allows for any symbol (as far as I am aware) to be used as the limiters in regular expressions.
Personally I tend to always use / as it is generally the standard in other languages. Some people will use other characters as the limiters if they are going to be using the / in the regular expression itself so that it doesn't have to be escaped.
Basically, if you use a particular character as the limiter of the expression, you will have to escape that character if it is used in the expression.