I don’t think the problem is that “([^\s]]+)” is capturing everything. Rather you are matching/capturing nothing. So you are just getting back the original string. My guess is that the the single line flag does not work with replace.
Try replacing "/^.?\[fpath:([^\s\]]+)]?.+$/s" with "/^.*?\[fpath:([^\s\]]+)]([^a]|[a])*/".
This will accomplish basically the same thing without requiring the single line flag.
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