Returns a 2-element array containing the beginning and ending
offsets (in characters) of the specified match.

When non-negative integer argument +n+ is given,
returns the starting and ending offsets of the <tt>n</tt>th match:

  m = /(.)(.)(\d+)(\d)/.match("THX1138.")
  # => #<MatchData "HX1138" 1:"H" 2:"X" 3:"113" 4:"8">
  m[0]        # => "HX1138"
  m.offset(0) # => [1, 7]
  m[3]        # => "113"
  m.offset(3) # => [3, 6]

  m = /(ん)(に)(ち)/.match('こんにちは')
  # => #<MatchData "んにち" 1:"ん" 2:"に" 3:"ち">
  m[0]        # => "んにち"
  m.offset(0) # => [1, 4]
  m[3]        # => "ち"
  m.offset(3) # => [3, 4]

When string or symbol argument +name+ is given,
returns the starting and ending offsets for the named match:

  m = /(?<foo>.)(.)(?<bar>.)/.match("hoge")
  # => #<MatchData "hog" foo:"h" bar:"g">
  m[:foo]         # => "h"
  m.offset('foo') # => [0, 1]
  m[:bar]         # => "g"
  m.offset(:bar)  # => [2, 3]

Related: MatchData#byteoffset, MatchData#begin, MatchData#end.
