preg interpretation question

Can someone interpret this preg string for me?

/[^a-z0-9\\/\\\\_.:-]/i

Here is what I can decipher.  the match is case insensitive, and it 
excludes all letters, and numbers, and the characters _ . : and - (that 
is underscore, period, colon, and dash).  The part I don't understand is 
\\/\\\\.  Sees to me to say backslash, slash, backslash, backslash.  Why 
the need for so many backslashes?