SED -- STREAM EDITOR

      sed 'command;command;command' somefile                see results
      sed 'command;command;command' somefile > newfile      save results
      .... | sed 'command;command;command' | ....           use in a pipeline
      sed -f sedcommands somefile > newfile                 commands are in file
                                                            somewhere else

   Commands:

      s   -- substitute strings
      y   -- translate characters
      d   -- delete lines
      !d  -- extract (keep) lines
      p   -- print lines

MOST COMMON PATTERN:   substitute every occurrence of a string "old" with "new"

      sed 's/old/new/g' somefile > newfile
      
The patterns in these commands use regular expressions for pattern matching:

      *       0 or more occurrences of previous character
      .       any character except newline
      [...]   any single occurrence of character in set
      [a-z]   any single occurrence of character in range
      [^...]  any single occurrence of character NOT in set (or range)
      ^       beginning of line
      $       end of line
      \       escapes the following character
      \{n,m\} range of occurrences, n and m are integers
      +       one or more occurrence of previous character
      ?       0 or more occurrences of preceding regular expression
      |       match either r.e. on left or r.e. on right
      ()      groups regular expressions

   EXAMPLES:
     /^M.*/   Line begins with capital M, 0 or more chars follow
     /..*/    At least 1 character long
     /^$/     The empty line
     ab|cd    Either ab or cd

Examples:

     sed 's/^Unix/UNIX(TM)/'  ...    The caret (^) matches the beginning of a
                                     line, so this only changes Unix if it is at
                                     the beginning of a line

     sed 's/:$//' ...                The dollar sign ($) matches the end of a 
                                     line, so this removes a colon only if it is
                                     at the end of a line.

More details in the sed examples: