[Boost-users] String algorithm? Removing non-alphanumerics from a string

Hi, Takeing an std::string as input, I need to return the same string with any characters not in the ranges "az", "AZ" and "09" being discarded. A colleague told me that a could use something in the string algorithm library, but I'm obviously not spotting what I need. I tried regex replace, but failed, as I'm not understanding regular expressions yet. Any and all suggestions would be gratefully received. Best wishes. Tim Burgess