perl -e '$_ = "test-string"; print "okay\n" if (/[-st]{5}/)'
# that should print "okay"

perl -e '$_ = "test-string"; print "okay\n" if (/[s\-t]{5}/)'
# should do the same

perl -e '$_ = "test-string"; print "okay\n" if (/[s-t]{5}/)'
# prints nothing: regex didn't match

perl -e '$_ = "test-string"; print "okay\n" if (/[t-s]{5}/)'
# prints an error message: "Invalid [] range "t-s" in regex...