in reply to Amusement in Auto-Increments

Bizzare!
Bizarre, but documented: 
       The auto-increment operator has a little extra builtin magic to it.  If
       you increment a variable that is numeric, or that has ever been used in
       a numeric context, you get a normal increment.  If, however, the
       variable has been used in only string contexts since it was set, and
       has a value that is not the empty string and matches the pattern
       "/^[a-zA-Z]*[0-9]*\z/", the increment is done as a string, preserving
       each character within its range, with carry:

           print ++($foo = '99');      # prints '100'
           print ++($foo = 'a0');      # prints 'a1'
           print ++($foo = 'Az');      # prints 'Ba'
           print ++($foo = 'zz');      # prints 'aaa'

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Re^2: Amusement in Auto-Increments
by cog (Parson) on Jul 22, 2009 at 20:52 UTC

    Using this bit of documentation, here's what happens:

  • 1zz

    • 1zz doesn't match "/^[a-zA-Z]*[0-9]*\z/"

    Rule #1 - gets converted to a number, 1, which is then incremented to 2.

  • zzz

    • zzz matches "/^[a-zA-Z]*[0-9]*\z/"

    Rule #2 - the last z is incremented to a (stays in the same range, which is a-z), carries one over; second z is incremented to a, carries one over; first z is incremented to a, carries one over, which gets turned into an a.

  • zz1

    • zz1 matches "/^[a-zA-Z]*[0-9]*\z/"

    Rule #2 - 1 is incremented to 2.

  • z3z

    • z3z doesn't match "/^[a-zA-Z]*[0-9]*\z/"

    Rule #1 - the value is converted to numeric; since there are no digits before the first non-digit, the numeric value is 0; that value is then incremented to 1.

  • zz9

    • zz9 matches "/^[a-zA-Z]*[0-9]*\z/"

    Rule #2 - 9 is incremented to 0, carries one over, and then the process with the two zs is the same as from above.

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