Clean up use of range-expressions in regexes

Range-expressions like [a-z] and [0-9] are widely used in regular expressions.
However, the POSIX spec says their meaning is locale-dependent; and glibc 2.28 apparently intends to implement this faithfully, which shall break range-expressions in many contexts.

This is likely to break shell-scripts using such ranges in glob; and may break other uses of regular expressions. We shall need to review our use of such ranges and aim to convert them, e.g. using [[:lower:]] for [a-z] and [[:digit:]] for [0-9], before too much of the world has gone over to glibc 2.28.