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

[Dr. Yuan Liu]

Instant Democracy wrote:
> 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.
>
> For lines which do not have ../.. in them, this is
> trivial, for example by regexp in sed, emacs etc.

It turns out to be trivial in both cases.

> The real problem is constructing a regular expression for the DIRNAME 
before the /..

In order to illustrate your question, I reconstructed test data to be 
non-trivial.  The solution is just one regexp with one loop in sed, 
':2dots; s;[^/]*/\.\./;;g; t2dots':

$ sed ':2dots; s;[^/]*/\.\./;;g; t2dots' <<EOM # test data ends with EOM
d01/dir.name/../dir/../file
d02/dir/../d2/../file.ext1
d03/d1/d2/../../file.ext2
EOM
d01/file
d02/file.ext1
d03/file.ext2

Just learned this loop trick from John L. a couple of days ago:-)

Yuan Liu