String#next

Returns the successor to self. The successor is calculated by incrementing characters.

The first character to be incremented is the rightmost alphanumeric: or, if no alphanumerics, the rightmost character:

'THX1138'.succ   # => "THX1139"
'<<koala>>'.succ # => "<<koalb>>"
'***'.succ       # => '**+'
'тест'.succ      # => "тесу"
'こんにちは'.succ  # => "こんにちば"

The successor to a digit is another digit, “carrying” to the next-left character for a “rollover” from 9 to 0, and prepending another digit if necessary:

'00'.succ # => "01"
'09'.succ # => "10"
'99'.succ # => "100"

The successor to a letter is another letter of the same case, carrying to the next-left character for a rollover, and prepending another same-case letter if necessary:

'aa'.succ # => "ab"
'az'.succ # => "ba"
'zz'.succ # => "aaa"
'AA'.succ # => "AB"
'AZ'.succ # => "BA"
'ZZ'.succ # => "AAA"

The successor to a non-alphanumeric character is the next character in the underlying character set’s collating sequence, carrying to the next-left character for a rollover, and prepending another character if necessary:

s = 0.chr * 3   # => "\x00\x00\x00"
s.succ        # => "\x00\x00\x01"
s = 255.chr * 3 # => "\xFF\xFF\xFF"
s.succ        # => "\x01\x00\x00\x00"

Carrying can occur between and among mixtures of alphanumeric characters:

s = 'zz99zz99' # => "zz99zz99"
s.succ         # => "aaa00aa00"
s = '99zz99zz' # => "99zz99zz"
s.succ         # => "100aa00aa"

The successor to an empty String is a new empty String:

''.succ # => ""

Related: see Converting to New String.