== Character Selectors

=== Character Selector

A _character_ _selector_ is a string argument accepted by certain Ruby methods.
Each of these instance methods accepts one or more character selectors:

- String#tr(selector, replacements): returns a new string.
- String#tr!(selector, replacements): returns +self+ or +nil+.
- String#tr_s(selector, replacements): returns a new string.
- String#tr_s!(selector, replacements): returns +self+ or +nil+.
- String#count(*selectors): returns the count of the specified characters.
- String#delete(*selectors): returns a new string.
- String#delete!(*selectors): returns +self+ or +nil+.
- String#squeeze(*selectors): returns a new string.
- String#squeeze!(*selectors): returns +self+ or +nil+.

A character selector identifies zero or more characters in +self+
that are to be operands for the method.

In this section, we illustrate using method String#delete(selector),
which deletes the selected characters.

In the simplest case, the characters selected are exactly those
contained in the selector itself:

  'abracadabra'.delete('a')   # => "brcdbr"
  'abracadabra'.delete('ab')  # => "rcdr"
  'abracadabra'.delete('abc') # => "rdr"
  '0123456789'.delete('258')  # => "0134679"
  '!@#$%&*()_+'.delete('+&#') # => "!@$%*()_"
  'тест'.delete('т')          # => "ес"
  'こんにちは'.delete('に')     # => "こんちは"

Note that order and repetitions do not matter:

  'abracadabra'.delete('dcab') # => "rr"
  'abracadabra'.delete('aaaa') # => "brcdbr"

In a character selector, these three characters get special treatment:

- A leading caret (<tt>'^'</tt>) functions as a "not" operator
  for the characters to its right:

    'abracadabra'.delete('^bc') # => "bcb"
    '0123456789'.delete('^852') # => "258"

- A hyphen (<tt>'-'</tt>) between two other characters
  defines a range of characters instead of a plain string of characters:

    'abracadabra'.delete('a-d') # => "rr"
    '0123456789'.delete('4-7')  # => "012389"
    '!@#$%&*()_+'.delete(' -/') # => "@^_"

    # May contain more than one range.
    'abracadabra'.delete('a-cq-t') # => "d"

    # Ranges may be mixed with plain characters.
    '0123456789'.delete('67-950-23') # => "4"

    # Ranges may be mixed with negations.
    'abracadabra'.delete('^a-c') # => "abacaaba"

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

    'abracadabra^'.delete('\^bc')   # => "araadara"
    'abracadabra-'.delete('a\-d')   # => "brcbr"
    "hello\r\nworld".delete("\r")   # => "hello\nworld"
    "hello\r\nworld".delete("\\r")  # => "hello\r\nwold"
    "hello\r\nworld".delete("\\\r") # => "hello\nworld"

=== Multiple Character Selectors

These instance methods accept multiple character selectors:

- String#count(*selectors): returns the count of the specified characters.
- String#delete(*selectors): returns a new string.
- String#delete!(*selectors): returns +self+ or +nil+.
- String#squeeze(*selectors): returns a new string.
- String#squeeze!(*selectors): returns +self+ or +nil+.

In effect, the given selectors are formed into a single selector
consisting of only those characters common to _all_ of the given selectors.

All forms of selectors may be used, including negations, ranges, and escapes.

Each of these pairs of method calls is equivalent:

  s.delete('abcde', 'dcbfg')
  s.delete('bcd')

  s.delete('^abc', '^def')
  s.delete('^abcdef')

  s.delete('a-e', 'c-g')
  s.delete('cde')
