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version::Internal - Perl extension for Version Objects


Overloaded version objects for all modern versions of Perl. This documents the internal data representation and underlying code for See version.pod for daily usage. This document is only useful for users writing a subclass of or interested in the gory details.

What IS a version

For the purposes of this module, a version "number" is a sequence of positive integer values separated by one or more decimal points and optionally a single underscore. This corresponds to what Perl itself uses for a version, as well as extending the "version as number" that is discussed in the various editions of the Camel book.

There are actually two distinct kinds of version objects:

Both of these methods will produce similar version objects, in that the default stringification will yield the version "Normal Form" only if required:

$v  = version->new(1.002);     # 1.002, but compares like 1.2.0
$v  = version->new(1.002003);  # 1.002003
$v2 = version->new("v1.2.3");  # v1.2.3

In specific, version numbers initialized as "Decimal Versions" will stringify as they were originally created (i.e. the same string that was passed to new(). Version numbers initialized as "Dotted-Decimal Versions" will be stringified as "Normal Form".

Decimal Versions

These correspond to historical versions of Perl itself prior to 5.6.0, as well as all other modules which follow the Camel rules for the $VERSION scalar. A Decimal version is initialized with what looks like a floating point number. Leading zeros are significant and trailing zeros are implied so that a minimum of three places is maintained between subversions. What this means is that any subversion (digits to the right of the decimal place) that contains less than three digits will have trailing zeros added to make up the difference, but only for purposes of comparison with other version objects. For example:

                                 # Prints     Equivalent to  
$v = version->new(      1.2);    # 1.2        v1.200.0
$v = version->new(     1.02);    # 1.02       v1.20.0
$v = version->new(    1.002);    # 1.002      v1.2.0
$v = version->new(   1.0023);    # 1.0023     v1.2.300
$v = version->new(  1.00203);    # 1.00203    v1.2.30
$v = version->new( 1.002003);    # 1.002003   v1.2.3

All of the preceding examples are true whether or not the input value is quoted. The important feature is that the input value contains only a single decimal. See also "Alpha Versions" for how to handle

IMPORTANT NOTE: As shown above, if your Decimal version contains more than 3 significant digits after the decimal place, it will be split on each multiple of 3, so 1.0003 is equivalent to v1.0.300, due to the need to remain compatible with Perl's own 5.005_03 == 5.5.30 interpretation. Any trailing zeros are ignored for mathematical comparison purposes.

Dotted-Decimal Versions

These are the newest form of versions, and correspond to Perl's own version style beginning with 5.6.0. Starting with Perl 5.10.0, and most likely Perl 6, this is likely to be the preferred form. This method normally requires that the input parameter be quoted, although Perl's after 5.8.1 can use v-strings as a special form of quoting, but this is highly discouraged.

Unlike "Decimal Versions", Dotted-Decimal Versions have more than a single decimal point, e.g.:

                                 # Prints
$v = version->new( "v1.200");    # v1.200.0
$v = version->new("v1.20.0");    # v1.20.0
$v = qv("v1.2.3");               # v1.2.3
$v = qv("1.2.3");                # v1.2.3
$v = qv("1.20");                 # v1.20.0

In general, Dotted-Decimal Versions permit the greatest amount of freedom to specify a version, whereas Decimal Versions enforce a certain uniformity. See also "New Operator" for an additional method of initializing version objects.

Just like "Decimal Versions", Dotted-Decimal Versions can be used as "Alpha Versions".

Decimal Alpha Versions

The one time that a Decimal version must be quoted is when a alpha form is used with an otherwise Decimal version (i.e. a single decimal point). This is commonly used for CPAN releases, where CPAN or CPANPLUS will ignore alpha versions for automatic updating purposes. Since some developers have used only two significant decimal places for their non-alpha releases, the version object will automatically take that into account if the initializer is quoted. For example Module::Example was released to CPAN with the following sequence of $VERSION's:

# $VERSION    Stringified
0.01          0.01
0.02          0.02
0.02_01       0.02_01
0.02_02       0.02_02
0.03          0.03

The stringified form of Decimal versions will always be the same string that was used to initialize the version object.

High level design

version objects provides an overloaded version object that is designed to both encapsulate the author's intended $VERSION assignment as well as make it completely natural to use those objects as if they were numbers (e.g. for comparisons). To do this, a version object contains both the original representation as typed by the author, as well as a parsed representation to ease comparisons. Version objects employ overload methods to simplify code that needs to compare, print, etc the objects.

The internal structure of version objects is a blessed hash with several components:

    bless( {
      'original' => 'v1.2.3_4',
      'alpha' => 1,
      'qv' => 1,
      'version' => [
    }, 'version' );

A faithful representation of the value used to initialize this version object. The only time this will not be precisely the same characters that exist in the source file is if a short dotted-decimal version like v1.2 was used (in which case it will contain 'v1.2'). This form is STRONGLY discouraged, in that it will confuse you and your users.


A boolean that denotes whether this is a decimal or dotted-decimal version. See is_qv.


A boolean that denotes whether this is an alpha version. NOTE: that the underscore can can only appear in the last position. See is_alpha.


An array of non-negative integers that is used for comparison purposes with other version objects.


In addition to the version objects, this modules also replaces the core UNIVERSAL::VERSION function with one that uses version objects for its comparisons. The return from this operator is always the stringified form as a simple scalar (i.e. not an object), but the warning message generated includes either the stringified form or the normal form, depending on how it was called.

For example:

package Foo;
$VERSION = 1.2;

package Bar;
$VERSION = "v1.3.5"; # works with all Perl's (since it is quoted)

package main;
use version;

print $Foo::VERSION; # prints 1.2

print $Bar::VERSION; # prints 1.003005

eval "use foo 10";
print $@; # prints "foo version 10 required..."
eval "use foo 1.3.5; # work in Perl 5.6.1 or better
print $@; # prints "foo version 1.3.5 required..."

eval "use bar 1.3.6";
print $@; # prints "bar version 1.3.6 required..."
eval "use bar 1.004"; # note Decimal version
print $@; # prints "bar version 1.004 required..."

IMPORTANT NOTE: This may mean that code which searches for a specific string (to determine whether a given module is available) may need to be changed. It is always better to use the built-in comparison implicit in use or require, rather than manually poking at class-VERSION> and then doing a comparison yourself.

The replacement UNIVERSAL::VERSION, when used as a function, like this:

print $module->VERSION;

will also exclusively return the stringified form. See Stringification for more details.

Usage question

Using modules that use

As much as possible, the module remains compatible with all current code. However, if your module is using a module that has defined $VERSION using the version class, there are a couple of things to be aware of. For purposes of discussion, we will assume that we have the following module installed:

package Example;
use version;  $VERSION = qv('1.2.2');
...module code here...
Decimal versions always work

Code of the form:

use Example 1.002003;

will always work correctly. The use will perform an automatic $VERSION comparison using the floating point number given as the first term after the module name (e.g. above 1.002.003). In this case, the installed module is too old for the requested line, so you would see an error like:

Example version 1.002003 (v1.2.3) required--this is only version 1.002002 (v1.2.2)...
Dotted-Decimal version work sometimes

With Perl >= 5.6.2, you can also use a line like this:

use Example 1.2.3;

and it will again work (i.e. give the error message as above), even with releases of Perl which do not normally support v-strings (see "What about v-strings" below). This has to do with that fact that use only checks to see if the second term looks like a number and passes that to the replacement UNIVERSAL::VERSION. This is not true in Perl 5.005_04, however, so you are strongly encouraged to always use a Decimal version in your code, even for those versions of Perl which support the Dotted-Decimal version.

Object Methods

Overloading has been used with version objects to provide a natural interface for their use. All mathematical operations are forbidden, since they don't make any sense for base version objects. Consequently, there is no overloaded numification available. If you want to use a version object in a Decimal context for some reason, see the numify object method.

For the subsequent examples, the following three objects will be used:

$ver   = version->new(""); # see "Quoting" below
$alpha = version->new("1.2.3_4"); # see "Alpha versions" below
$nver  = version->new(1.002);     # see "Decimal Versions" above


Because of the nature of the Perl parsing and tokenizing routines, certain initialization values must be quoted in order to correctly parse as the intended version, especially when using the qv() operator. In all cases, a floating point number passed to version->new() will be identically converted whether or not the value itself is quoted. This is not true for qv(), however, when trailing zeros would be stripped on an unquoted input, which would result in a very different version object.

In addition, in order to be compatible with earlier Perl version styles, any use of versions of the form 5.006001 will be translated as v5.6.1. In other words, a version with a single decimal point will be parsed as implicitly having three digits between subversions, but only for internal comparison purposes.

The complicating factor is that in bare numbers (i.e. unquoted), the underscore is a legal Decimal character and is automatically stripped by the Perl tokenizer before the version code is called. However, if a number containing one or more decimals and an underscore is quoted, i.e. not bare, that is considered a "Alpha Version" and the underscore is significant.

If you use a mathematic formula that resolves to a floating point number, you are dependent on Perl's conversion routines to yield the version you expect. You are pretty safe by dividing by a power of 10, for example, but other operations are not likely to be what you intend. For example:

$VERSION = version->new((qw$Revision: 1.4)[1]/10);
print $VERSION;          # yields 0.14
$V2 = version->new(100/9); # Integer overflow in decimal number
print $V2;               # yields something like

Perl 5.8.1 and beyond will be able to automatically quote v-strings but that is not possible in earlier versions of Perl. In other words:

$version = version->new("v2.5.4");  # legal in all versions of Perl
$newvers = version->new(v2.5.4);    # legal only in Perl >= 5.8.1


This module is specifically designed and tested to be easily subclassed. In practice, you only need to override the methods you want to change, but you have to take some care when overriding new() (since that is where all of the parsing takes place). For example, this is a perfect acceptable derived class:

package myversion;
use base version;
sub new { 
    my $obj;
    # perform any special input handling here
    $obj = $self->SUPER::new($n);
    # and/or add additional hash elements here
    return $obj;

See also version::AlphaBeta on CPAN for an alternate representation of version strings.

NOTE: Although the qv operator is not a true class method, but rather a function exported into the caller's namespace, a subclass of version will inherit an import() function which will perform the correct magic on behalf of the subclass.


qv - Dotted-Decimal Version initialization operator


John Peacock <>