Data::Decrement - Provide extra magic logic for auto-decrement


This document describes version 0.002 of Data::Decrement (from Perl distribution Data-Decrement), released on 2019-01-26.


 use Data::Decrement 'decr';

 print decr("b00"); # prints "a99"


Perl's auto-increment operator (++) has some convenience feature built in. Quoting perlop:

 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"

 "undef" is always treated as numeric, and in particular is changed to 0 before
 incrementing (so that a post-increment of an undef value will return 0 rather
 than "undef").

 The auto-decrement operator is not magical.

This module provides the decr() function to do the decrement equivalent, although it is not exactly the reverse of the increment operation. In general, the rule is that decr(++$a) should return the same value as the original $a before the auto-increment, with a couple of exception.

  • Positive integers are decremented as string

    Positive integers, including those with zero prefix, are decremented as string.

     print decr(-123);            # prints "-124", treated as number
     print decr(123);             # prints "122", treated as string
     print decr(100);             # prints "099", treated as string

    "undef" like in auto-increment is treated as number 0.

     print decr(undef);            # prints "-1", treated as number
  • Decrementing is not done when leftmost digit is already "A", "a", or 0

    When carrying over to the left-most digit, and the digit is already "A", "a", or "0", decrementing is not done. The original value is returned and a warning "Cannot decrement '<VALUE>'" is issued. Examples:

     print decr(0);               # prints "0", warns "Cannot decrement '0'"
     print decr("a1");            # prints "a0"
     print decr("b0");            # prints "a9"
     print decr("a0");            # prints "a0", warns "Cannot decrement 'a0'"
     print decr("bZz0");          # prints "bZy9"
     print decr("bZa0");          # prints "bYz9"
     print decr("bAa0");          # prints "aZz9"
     print decr("aAa0");          # prints "aAa0", warns "Cannot decrement 'aAa0'"




 decr($val) => $dec_val

Accept a value and return decremented value. If $val matches the pattern /^[a-zA-Z]*[0-9]*\z/, it will decremented as a string (note that positive integers match this pattern). Otherwise, it will be decremented numerically. undef is regarded as numeric 0.

Will return the original value and emit a warning if cannot decrement a value.


Please visit the project's homepage at


Source repository is at


Please report any bugs or feature requests on the bugtracker website

When submitting a bug or request, please include a test-file or a patch to an existing test-file that illustrates the bug or desired feature.


++ in perlop

dec-pl in App::IncrementUtils


perlancar <>


This software is copyright (c) 2019 by

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.