What does the syntax on this shell command with extglob mean?

Question

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.

Answer 1

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.

Answer 2

From the bash(1) man page:

          +(pattern-list)
                 Matches one or more occurrences of the given patterns

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

Answer 3

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

Here, I can select case insensitive names:

#!/bin/zsh
setopt extendedglob

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