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Re: NON-trivial regular expression problem (could not find on google)
From: |
Larry Clapp |
Subject: |
Re: NON-trivial regular expression problem (could not find on google) |
Date: |
Sat, 18 Jan 2003 19:27:24 -0500 |
User-agent: |
slrn/0.9.7.4 (Linux) |
In article <7606630f.0301181219.60384da2@posting.google.com>, 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.
>
> The real problem is constructing a regular expression for the
> DIRNAME before the /..
>
> This DIRNAME can be described as a string that contains neither
> / not double-dot but anything else. Perhaps I am overlooking
> something else about DIRNAME.
Yes. "../.." doesn't matter. Find any pathname component
followed by ".." and remove both. Continue until no more ".."'s
exist in the string. In Perl:
$_ = "d1/d2/../../file.ext";
do {} while (s#[^/]+/\.\./##g);
print $_;
In Common Lisp:
(defun split (s delim)
(let ((start-at 0))
(nconc
(loop for c across s
and i upto (length s)
nconc (when (eql c delim)
(prog1
(list (subseq s start-at i))
(setf start-at (1+ i)))))
(list (subseq s start-at)))))
(defun join (list delim)
(let ((res (car list))
(delim (make-string 1 :initial-element delim)))
(dolist (el (cdr list))
(setf res (concatenate 'string res delim el)))
res))
(defun normalize-path (path)
(let ((names (split path #\/))
(stack nil))
(dolist (name names)
(cond ((string= name "..") (pop stack))
(t (push name stack))))
(join (reverse stack) #\/)))
Testing:
(let ((paths '("dir.name/../dir/../file"
"dir/../d2/../file.ext"
"d1/d2/../../file.ext"
"1234/../2345/../3546/3456/3456/../../asdf/asdf/file")))
(dolist (path paths)
(print (normalize-path path))))
-> "file"
-> "file.ext"
-> "file.ext"
-> "3546/asdf/asdf/file"
=> NIL
> [...]
> sigfile - you are welcome to use it or modify without changing
> its thrust.
Please drop this 3k sigfile. Thank you.
-- Larry
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- NON-trivial regular expression problem (could not find on google), Instant Democracy, 2003/01/18
- Re: NON-trivial regular expression problem (could not find on google), Harry Putnam, 2003/01/18
- Re: NON-trivial regular expression problem (could not find on google), Kai Großjohann, 2003/01/18
- Re: NON-trivial regular expression problem (could not find on google), AW, 2003/01/18
- Re: NON-trivial regular expression problem (could not find on google), Edi Weitz, 2003/01/18
- Re: NON-trivial regular expression problem (could not find on google),
Larry Clapp <=
- Re: NON-trivial regular expression problem (could not find on google), Dr. Yuan Liu, 2003/01/18
- Re: NON-trivial regular expression problem (could not find on google), John W. Krahn, 2003/01/18
- Re: NON-trivial regular expression problem (could not find on google), William Park, 2003/01/18
- Re: NON-trivial regular expression problem (could not find on google), Peter J. Acklam, 2003/01/19
- Re: NON-trivial regular expression problem (could not find on google), Bruce Barnett, 2003/01/24