5/12    Perl question, I want to replace nnn with xxx, ie, \nnnn -> \nxxx
        (that's a backslash followed by n, not a newline), s/nnn/xxx/
        gives me \xxxn!! What to do? I want the replace to change everything
        except a literal \n that should stay as is...
        \_ I think s/(\\n)?nnn/$1xxx/ will do it.  That's "match an optional
           backslash-n and nnn and replace nnn with xxx".
           \_ Alternatively, s/(?<!\\)nnn/xxx/ explicitly matches any "nnn"
              that isn't preceded by a backslash.  You may or may not find
              this more intuitive.
                \_ ?<! is the syntax? how weird.
                   \_ It's based on the older syntax of ?!, which looks ahead
                      in the string: /foo(?!bar)/ matches foo but not foobar.
                      When they added the ability to look behind, matching
                      bar but not foobar, /(?<!foo)bar/ seemed like the least
                      bad choice.

                      Yes, it's a mess.  The tentative perl6 syntax for this
                      is /<!after foo>bar/, where <!after foo> is read "not
                      after foo" and means "assert that the current position
                      in the string is not immediately after something that
                      matches foo".  --mconst
                      \_ I must say the introduction of actual English keywords
                         (in regexps, no less!) instead of line noise is a
                         philosophical novelty for Perl.  Or maybe they added
                         so much functionality that the pigeonhole principle
                         hit them pretty hard, and they had no choice. -- ilyas
                         \_ Heh.  You might actually enjoy reading some of
                            Larry Wall's rationale for the new regexp syntax;
                            it includes quotes like:

                                It should not be considered acceptable to
                                define new constructs that contain a plethora
                                of punctuation, but we've become accustomed
                                to constructs like (?<=...) and (??{...}) and
                                [\r\n\ck\p{Zl}\p{Zp}], so we don't complain.

                            Check out http://csua.org/u/7af starting at "First, let
                            me enumerate some of the things that are wrong
                            with current regex culture."  --mconst
                            \_ But I *like* line noise!  It makes me look
                               brilliant at work.