In Building paths robustly,

A solution is proposed that includes:

shopt -s extglob; dir="${dir//+(\/)//}"

Can someone explain (parse) that for me? I understand what it's doing, but not how the syntax works.

3 Answers 3


dir="${dir//+(\/)//}" is using the ${PARAMETER/PATTERN/STRING} expansion. Breaking it down in this case (using a syntax reminiscent of Perl's /x regex modifier, not valid Bash syntax):

${       # start expansion
  dir    # the parameter being expanded
  /      # separates parameter from pattern
  /      # double slash means replace all instead of replace first
  +(\/)  # the pattern we're looking for
  /      # separates pattern from replacement
  /      # the replacement text
}        # end expansion

With extglob enabled, +(PATTERN) means one or more occurrences of PATTERN. The pattern \/ matches a slash (the backslash is to indicate that this isn't the slash that separates the pattern and the replacement text), so +(\/) matches one or more / characters.

  • Rant: I have enough trouble with regexes to start with. Now I have to learn another syntax! Thank you.
    – Joe
    Nov 1, 2011 at 8:42

From the bash(1) man page:

                 Matches one or more occurrences of the given patterns

So, it is like the + regex operator, applied to the pattern within the parens.

  • Some examples with do-s and-don't-s -- Bash Extended Globbing ...(I've now also added this same link to the page you referred to :)....
    – Peter.O
    Oct 30, 2011 at 4:13
  • The first double slash // is as per the normal multiple replace expansion syntax... The second double slash // is made up of two syntactically quite different elements... The first one is the single slash which delimits the search patern from the replacement... the second (final) slash it the actual value of the replacement... The regex is explained in the link and man page... A point of note, is that you can actually mix normal globs with the regex syntax...
    – Peter.O
    Oct 30, 2011 at 4:37
  • Your link appears to have a typo in it. I found linuxjournal.com/content/bash-extended-globbing . I wanted to select your answer too.
    – Joe
    Nov 1, 2011 at 8:39

Depends on the shell. I use zsh, which has quite a lot of features with it:

Here, I can select case insensitive names:

setopt extendedglob

for f in (#i)*.(jpeg|jpg|png); do
        mogrify -scale 1280x1280 $f
        exiv2 rm $f
        mv $f $f:l
    } &

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