I realize that on Fedora 27, the output of `darktable --version` is:
"this is darktable darktable-2.2.5-4.fc27"
This is different from the expected output in our plug-in ("this is
darktable 2.2.5"). I assume this version string can be customized and
distribution packagers will use the capability. So a regular expression,
in a slightly more flexible fashion would be better. I still assume that
the version string with start with "this is darktable", but then I
accept any non-number string until I reach a common major.minor.patch
version-looking string.
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