Assignment 6 (4%)

• Write a simple Python program that acts similar to a stripped-down version of the UNIX stream editor sed:
  • Any command line argument that does not start in a - (dash) is seen as an input file name
  • Any command that starts in a dash is an editor command
  • The program reads all lines from all input files (in the specified order, if no input files is present, read stdin), executes all editor commands on each line, and prints it to the standard output.
• Editor commands:
  • -g<pattern>: Grep for the pattern (a regular expression string), i.e. print only lines in which a string matching the pattern occurs
  • -s<pattern>/<repl>: Substitute all strings matching <pattern> with <repl> before printing
• Each option can occur multiple times, the effect is the same as if the input would be piped through a sequence of programs:

     ssed.py -gHello -syou/me -gHolla infile
  

  is the same as

     ssed.py -gHello infile | ssed.py -syou/me | ssed.py -gHolla 
  

  i.e. the modifications and selections are performed sequentially on a per-line basis
• Additional explanation for multiple options/multiple input files:

  ssed.py <options1> file1 <options2> file2 file3 <options3>
  

  should print the same result as

  cat file1 file2 file3 | ssed.py <options1>| ssed.py <options2>| ssed.py <options3>
  

  i.e. order of files is significant, order of options is significant, but how files and options are intermingeled is not.