Description of problem:
Using a regular expression range like [C-a] works fine if compiled with
regcomp() with just the REG_EXTENDED flag, but if the REG_ICASE flag is added
too, regcomp() returns an error "Invalid range end".

Testing other ranges with REG_ICASE reveals:
    [A-Z^-z] is invalid: Invalid range end (11)
    [A-Z^_`a-z] is ok
    [C-a] is invalid: Invalid range end (11)
    [C-f] is ok
    [_-a] is invalid: Invalid range end (11)
    [<-a] is ok
    [z-{] is ok

It appears that regcomp() is capitalizing the range if the REG_ICASE flag is
used, thus [C-a] becomes [C-A] and since A comes before C, the range is invalid.
 Likewise, in locales that match ASCII, ^ becomes before z, but after Z, so
[A-Z^-z] becomes invalid, and _ comes after A but before a, so [_-a] becomes
invalid.

If this is not considered a bug, then at the very least, the regex(3) man page
should note the side-effects of using REG_ICASE.

Version-Release number of selected component (if applicable):
glibc-2.3.4-2.43

How reproducible:
every time

Steps to Reproduce:
1. compile a regex range [C-a] or [_-a] with regcomp() and REG_ICASE flag
  
Actual results:
regcomp() returns an "Invalid range end" error

Expected results:
regcomp() is consistent with or without REG_ICASE flag

Additional info:
If this is considered not a bug, then please update the regcomp(3) man page to note the side effects of using REG_ICASE.