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If you're asking specifically what the regex re.search(r'(?<=\[)(.*?)(?=\])' does, it's finding strings that are surrounded by square braces and capturing the content inside the braces.

• (?<=\[) is a positive lookbehind, asserting that before this match, there's a square opening brace [
• (.*?) is a capture group that selects any characters any number of times, but the ? quantifier means it captures as few as possible. This means it won't usually capture the closing square brackets ahead of it; if you've got a[s]df[ghj]kl, it'll capture s and ghj instead of s]df[ghj.
• (?=\]) is a positive lookahead, asserting that after this match, there's a square closing brace ]

See it in action here!

I can sort of guess at the rest of the script, which looks like it's searching for symbolic links ("shortcuts") to something. The presence of dir {} /al makes me think this script is supposed to be run on Windows. subprocess.check_output() is going to run the command string inside of it, and then pipe its (possibly multiline) output to the regex, which is then going to use .group() to create a tuple of the matches it finds.