Autoincrement and Autodecrement (Programming Perl)

3.3. Autoincrement and Autodecrement

The ++ and -- operators work as in C. That is, when placed before a variable, they increment or decrement the variable before returning the value, and when placed after, they increment or decrement the variable after returning the value. For example, $a++ increments the value of scalar variable $a, returning the value before it performs the increment. Similarly, --$b{(/(\w+)/)[0]} decrements the element of the hash %b indexed by the first "word" in the default search variable ($_) and returns the value after the decrement.[2]

[2]Okay, so that wasn't exactly fair. We just wanted to make sure you were paying attention. Here's how that expression works. First the pattern match finds the first word in $_ using the regular expression \w+. The parentheses around that cause the word to be returned as a single-element list value because the pattern match is in a list context. The list context is supplied by the list slice operator, (...)[0], which returns the first (and only) element of the list. That value is used as the key for the hash, and the hash entry (value) is decremented and returned. In general, when confronted with a complex expression, analyze it from the inside out to see what order things happen in.

The autoincrement operator has a little extra built-in magic. 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 only been used in string contexts since it was set, has a value that is not the null string, and matches the pattern /^[a-zA-Z]*[0-9]*$/, the increment is done as a string, preserving each character within its range, with carry:

print ++($foo = '99');      # prints '100'
print ++($foo = 'a0');      # prints 'b1'
print ++($foo = 'Az');      # prints 'Ba'
print ++($foo = 'zz');      # prints 'aaa'

As of this writing, magical autoincrement has not been extended to Unicode letters and digits, but it might be in the future.

The autodecrement operator, however, is not magical, and we have no plans to make it so.