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I've some notes of useful regular expressions and one that I always use is the following:

echo '/home/user/folder/file.txt' | sed -E 's/[\\\/][^\\\/]*$//g'

The result that I get from this regular expression is the path of the parent folder /home/user/folder. I understand the basics of regular expressions with:

\s          # all white space
\S          # no white space
.           # all chars       
\.          # period
+           # sequence of once or more
{5}         # sequence of delimited interval 
*           # sequence of zero or more
?           # sequence of once or none
[0-9]       # any sequence of number
[a-z]       # any sequence of letter 
[^x-y]      # no sequence of letter 
^           # beginning
$           # ending

However, I haven't managed to figure out what is the meaning of [\\\/] and [^\\\/] in the case of the regular expression from my example. How does it work?

raylight
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  • Why don't you use what you understand and instead say sed 's#\(.*\)/.*#\1#' or sed 's#/[^/]*$##', or use the dirname utility? – Kusalananda Apr 15 '22 at 05:51
  • @Kusalananda Why is sed 's#\(.*\)/.*#\1#' and sed 's#/[^/]*$##' better than sed 's/[/][^/]*$//'? I made this question because I was struggling to understand that regular expression. It wasn't so much about solving the problem of getting dirname. Multiple times I made a mess on bash code because I was using dirname of dirname of dirname... The code can get ugly like that. – raylight Apr 15 '22 at 06:15
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    Ah, then I misunderstood why you included the list of regular expression features that you understood. And I also misunderstood what the end goal was. I thought you wanted to strip off the last part of a pathname. Note that the given regular expression does not help with stripping off arbitrary components of the pathname either, just like dirname does not help with that. In the zsh shell, it would be a simple matter of using $pathname:h, $pathname:h:h etc. – Kusalananda Apr 15 '22 at 06:30
  • @Kusalananda The pattern [\\\/] was out of it... I missed the concept that [abc] is a or b or c as explained in the accepted answer... So I wasn't understanding that it was just \ or / in the end. – raylight Apr 15 '22 at 06:33

1 Answers1

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[\\\/] contains an escaped \ and an escaped / (escaping this character is not necessary here). Like [abc] matches a or b or c, [\\\/] matches \ or /.

[^\\\/] is somewhat similar but ^ is special at the beginning of []: it negates the meaning. [^\\\/] matches any character other than \ or /.

[\\\/][^\\\/]*$ matches \ or /, then zero or more other characters till the end of the line. Your s command replaces the matched string with nothing. The whole sed command removes the last \ or the last / (whichever occurs later in the line) along with everything that follows in the line.

Notes:

  • -E is not needed for this particular command to work.
  • g is not needed (you cannot find more than one end of the line in a line).
  • (already noted) Escaping / inside [] is not needed. (Escaping / outside of [] is not needed in general; it's often needed because people particularly choose / as the delimiter in s/…/…/, but it can be another character, e.g. s|…|…|.)
  • Your command seems to be "universal" in a sense it removes the last component from Unix pathnames (components separated by /) and from DOS/Windows pathnames (components separated by \). But…
  • \ may appear in a Unix pathname. If it does then your sed command may give you an unexpected result. A newline character is also allowed.
  • / is a valid pathname and its parent directory is /. Your sed command yields an empty string though.
  • If dir is a directory then /path/to/dir/ is equivalent to /path/to/dir, but your sed command will yield /path/to/dir and /path/to respectively.
Kamil Maciorowski
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  • Cool, I actually didn't know that [abc] would make a or b or c... Most of the time I use ranges like [a-z] and [0-9]... So in the end the \ is more about windows users and sed -E 's/[\/][^\/]*$//g' would be enough to make it work on Linux... With the limitations that you mentioned though. Thanks! – raylight Apr 15 '22 at 05:19
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    @raylight Even sed 's/[/][^/]*$//' should do. – Kamil Maciorowski Apr 15 '22 at 05:21
  • I see... I'd be more like the case in sed 's/\/[^/]*\/[^/]*$//g'. Going back two folders now... That ends up being better than using dirname multiple times in my opinion... – raylight Apr 15 '22 at 05:39
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    If you want to mangle Unix pathnames, consider basename(1), dirname(1), or perhaps the builtins to your shell. – vonbrand Apr 15 '22 at 19:17