Matches operator doesn’t work with some hex digits

I don't know why the matches operator doesn't find some hex digits. If I do something like: http matches "\xff"

I get no packets. This doesn't happen with hex digit which start by a number.

The funny is that If I use contains: http contains "\xff"

I get the packet I'm looking for.

The problem is that I need to use matches operator to set some regex in perl like: "http matches "\xff..(\xef|\xff)+"

I'm using Wireshark 1.6.4 Any ideas? Thank you so much