Re: NON-trivial regular expression problem (could not find on google)

[Edi Weitz]

democrat@india.com (Instant Democracy) writes:

> Regular expression facilities are slightly varied in
> sed
> awk
> lisp
> emacs-lisp
> Therefore these newsgroups can all contribute to the discussion.
> 
> A frequent problem involves simplifying a pathname. The string format we
> can expect to encounter is covered by the following three examples:
> 
> "dir.name/../dir/../file"
> "dir/../d2/../file.ext"
> "d1/d2/../../file.ext"
> 
> The "" are part of the string, and not just string delimiters. These
> strings are inside regular text on the line. The paths are never
> absolute so that you will not encounter "/d1/file.ext".
> 
> The task is to eliminate patterns such as 
>     DIRNAME/../
> from the path because they are redundant.

In Lisp on a Unix system you would use the function TRUENAME provided
that the file in question exists, see

  <http://www.lispworks.com/reference/HyperSpec/Body/f_tn.htm>.

If I understand your problem description correctly this can't be
solved with regular expressions. Or, to be more exact, if the nesting
of "../" parts can be arbitrarily deep you won't be able to solve your
problem with a _single_ application of _one_ regular expression.

However, it should be easy to do this with _repeated_ applications of
a regular expression unless I missed something:

  perl -le 'while (<>) { 1 while s#[^/]+/\.\./##; print }'

Edi.