zsh mystery variable expansion - zsh

Question

I found this in prezto source code:
# Set the command name, or in the case of sudo or ssh, the next command.
local cmd="${${2[(wr)^(*=*|sudo|ssh|-*)]}:t}"
I've been reading zsh doc a lot but getting nowhere close to what this is about. In experimentation on the shell itself it seems to indicate the [] is some arithmetic thing, which makes sense, but I don't see the part that explains how (w) is supposed to work. It seems to be some magical operator that applies to the math expression...
slu#ubuntu-sluvm ~/.zprezto ❯❯❯ VAR="one two three four"
slu#ubuntu-sluvm ~/.zprezto ❯❯❯ echo ${VAR[2]}
n
slu#ubuntu-sluvm ~/.zprezto ❯❯❯ echo ${VAR[(w)2]}
two
slu#ubuntu-sluvm ~/.zprezto ❯❯❯ echo ${VAR[(w)]}
zsh: bad math expression: empty string
slu#ubuntu-sluvm ~/.zprezto ❯❯❯

Answer 1

It looks pretty messy at first glance, but once you break it into its parts it's quite simple. This is an example of parameter expansion and extended globbing support in ZSH. If you look higher up in the function from which this code sample is, you'll see they set:
emulate -L zsh
setopt EXTENDED_GLOB
So now let's break apart the line you have there:
${
${
2[ # Expand the 2nd argument
(wr) # Match a word
^(*=*|=|sudo|ssh|-*) # Do not match *=*, =, sudo, ssh, or -*
]
}
:t} # If it is a path, return only the filename
You can test this by creating a sample script like this:
#!/bin/zsh
emulate -L zsh
setopt EXTENDED_GLOB
echo "${$1[(wr)^(*=*|sudo|ssh|-*)]}:t}" # changed 2 to 1, otherwise identical
Here's what it outputs:
$ ./test.sh '/bin/zsh'
zsh
$ ./test.sh 'sudo test'
test
$ ./test.sh 'sudo --flag test'
test
$ ./test.sh 'ssh -o=value test'
test
$ ./test.sh 'test'
test
For more information, see the documentation on expansion and csh-style modifiers.