ruby count 函数的一个用法，不知道是什么意思
帮忙解释一下如下代码含义：
a = "hello world"
a.count "hello", "^l" #=> 4
以下是官方文档中的解释：
http://ruby-doc.org/core-2.3.0/String.html#method-i-count
count([other_str]+) → fixnum
Each other_str parameter defines a set of characters to count. The intersection of these sets defines the characters to count in str. Any other_str that starts with a caret ^ is negated. The sequence c1-c2 means all characters between c1 and c2. The backslash character \ can be used to escape ^ or - and is otherwise ignored unless it appears at the end of a sequence or the end of a other_str.
a = "hello world"
a.count "lo" #=> 5
a.count "lo", "o" #=> 2
a.count "hello", "^l" #=> 4
a.count "ej-m" #=> 4
"hello^world".count "\\^aeiou" #=> 4
"hello-world".count "a\\-eo" #=> 4
c = "hello world\\r\\n"
c.count "\\" #=> 2
c.count "\\A" #=> 0
c.count "X-\\w" #=> 3
