The regexp  [^f]  matches any charecter exept "f" , i.e. "f" is
  avoided.   Now: I want to avoid "foo", but [^foo] doesn't do the
  job.  So how do I avoid "foo" ( or a string ) in regexp ??

  hans

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The most straightforward way I know of is to use separate regexps
for the negatives:

# Not foo
(! /foo/)

# foo followed by anything but bar
(/foo/ && ! /foo bar/)

# foo followed by (anything but bar or club) followed by baz
(/foo .* baz/ && ! /foo (bar|club)/)

For complicated constructs, this can add up quickly.  In Larry's last
post on this, he implied that he would consider adding negative
expressions, but certainly did not commit to it.

This may qualify as a FAQ soon (if it isn't there already -- I didn't
notice it).
--

"When the only tool you have is Perl, the whole | "Hooray for snakes!"
 world begins to look like your oyster." -- Me  |  -- The Simpsons (29 Apr 93)
------- "We _love_ being politically Koreshed" -- Steve Taylor, "Smug" -------

To replace every "bar" by "baz", unless it is followed by "foo":

  s/(bar)/$1\000/g;            # Put a marker after every bar.
  s/\000(foo)/$1/g;            # Remove the marker, if followed by foo.
  s/bar\000/baz/g;             # Replace every bar that is still followed
                               # by a marker.

To replace every "bar.*foo" by "baz", where the string that is matched
by '.*' does not contain any "foo":

  s/(foo)/\000$1/g;            # Put a marker before every foo.
  s/bar[^\000]*\000foo/baz/g;  # Replace every bar[<nomarker>]*<marker>foo .
  s/\000//g;                   # Remove remaining markers.

To check whether $_ contains some "bar" that is not (immediately)
followed by "foo":

  s/(bar)/$1\000/g;            # Put a marker after every bar.
  s/\000(foo)/$1/g;            # Remove the marker, if followed by foo.
  m/\000/;                     # Check whether there is a marker left.

Another possibility:

  s/barfoo/foo/g;              # Remove every bar that is followed by foo.
  m/bar/;                      # Check whether there is a bar left.

Note that the following command _does_something_different_:

  m/bar/ && ! m/barfoo/;       # Check that there is a bar and that
                               # *no* bar is followed by foo.

It will fail if the input contains both a bar that is followed by foo
and a bar that is *not* followed by foo.

--
Uwe Waldmann, Max-Planck-Institut fuer Informatik
Im Stadtwald, D-66123 Saarbruecken, Germany