converting to hex

Hi,

I am trying to write a function that will allow a postgres to convert a
text string into a hex value but there doesnt seem a function to do it,
only one i could find is to_hex(number) example of what i have done is

   update table1
   Set field2 = encode((select md5('field1')),'hex')
   where new."primary" = "primary";

any help would be greatfully received