preg_replace, exact opposite of a preg_match

I need to do a preg_replace for the exact opposite of this preg_match regular expression:

preg_match('#^(\w+/){0,2}\w+\.\w+$#', $string);

So I need to replace all strings that are not valid with an empty string -> ''

So it needs to remove the first / and last / if found, and all non-valid characters, that is the only valid characters are A-Z, a-z, 0-9, _, ., and / (if it's not the fi开发者_StackOverflowrst or last characters of the string).

How can I accomplish this with the preg_replace?

Thanks :)

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preg_replace('#^/|/$|[^A-Za-z0-9_./]#D', '', $subject);