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This expression determines whether a directory called '''C:''' exists and--if it does--verifies that it is '''not''' a subdirectory called "'''C:'''" deliberately created by some wag for the sole purpose of defeating the check. Since, of course, one occasionally encounters a Windows system that does not feature a '''C:''' drive, the expression can be generalized thus:
'''join "", map { -d $_ && ! -d "./$_" } map { chr( $_
The inner '''map''' returns the set '''"A" .. "Z"'''; the middle '''map''' converts this to the set '''"A:" .. "Z:"'''; and the outer map applies the logical conjunct to each member of the colon-embellished set, the results of those 26 logical evaluations being concatenated by the '''join'''.
which actually applies the preceding logical test to the complete set '''A:''' through '''Z:''' of possible drives. Each test that succeeds returns a "1" while each that fails returns the empty string. Thus, if one or more of the drives exists, the return value will be a nonzero number, albeit possibly as large as 11111111111111111111111111.
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