Returns the total number of characters in +self+ that are specified by the given selectors.

For one 1-character selector,
returns the count of instances of that character:

  s = 'abracadabra'
  s.count('a') # => 5
  s.count('b') # => 2
  s.count('x') # => 0
  s.count('')  # => 0

  s = 'よろしくお願いします'
  s.count('よ')  # => 1
  s.count('し')  # => 2

For one multi-character selector,
returns the count of instances for all specified characters:

  s = 'abracadabra'
  s.count('ab')     # => 7
  s.count('abc')    # => 8
  s.count('abcd')   # => 9
  s.count('abcdr')  # => 11
  s.count('abcdrx') # => 11

Order and repetition do not matter:

  s.count('ba')   == s.count('ab') # => true
  s.count('baab') == s.count('ab') # => true

For multiple selectors,
forms a single selector that is the intersection of characters in all selectors
and returns the count of instances for that selector:

  s = 'abcdefg'
  s.count('abcde', 'dcbfg') == s.count('bcd') # => true
  s.count('abc', 'def')     == s.count('')    # => true

In a character selector, three characters get special treatment:

- A caret (<tt>'^'</tt>) functions as a _negation_ operator
  for the immediately following characters:

    s = 'abracadabra'
    s.count('^bc') # => 8  # Count of all except 'b' and 'c'.

- A hyphen (<tt>'-'</tt>) between two other characters defines a _range_ of characters:

    s = 'abracadabra'
    s.count('a-c') # => 8  # Count of all 'a', 'b', and 'c'.

- A backslash (<tt>'\'</tt>) acts as an escape for a caret, a hyphen,
  or another backslash:

    s = 'abracadabra'
    s.count('\^bc')           # => 3  # Count of '^', 'b', and 'c'.
    s.count('a\-c')           # => 6  # Count of 'a', '-', and 'c'.
    'foo\bar\baz'.count('\\') # => 2  # Count of '\'.

These usages may be mixed:

  s = 'abracadabra'
  s.count('a-cq-t') # => 10  # Multiple ranges.
  s.count('ac-d')   # => 7   # Range mixed with plain characters.
  s.count('^a-c')   # => 3   # Range mixed with negation.

For multiple selectors, all forms may be used, including negations, ranges, and escapes.

  s = 'abracadabra'
  s.count('^abc', '^def') == s.count('^abcdef') # => true
  s.count('a-e', 'c-g')   == s.count('cde')     # => true
  s.count('^abc', 'c-g')  == s.count('defg')    # => true

Related: see {Querying}[rdoc-ref:String@Querying].
