Params::Validate - Validate method/function parameters


  use Params::Validate qw(:all);

  # takes named params (hash or hashref)
  sub foo
      validate( @_, { foo => 1, # mandatory
                      bar => 0, # optional

  # takes positional params
  sub bar
      # first two are mandatory, third is optional
      validate_pos( @_, 1, 1, 0 );

  sub foo2
      validate( @_,
                { foo =>
                  # specify a type
                  { type => ARRAYREF },

                  bar =>
                  # specify an interface
                  { can => [ 'print', 'flush', 'frobnicate' ] },

                  baz =>
                  { type => SCALAR,   # a scalar ...
                    # ... that is a plain integer ...
                    regex => qr/^\d+$/,
                    callbacks =>
                    { # ... and smaller than 90
                      'less than 90' => sub { shift() < 90 },

  sub with_defaults
       my %p = validate( @_, { foo => 1, # required
                               # $p{bar} will be 99 if bar is not
                               # given.  bar is now optional.
                               bar => { default => 99 } } );

  sub pos_with_defaults
       my @p = validate( @_, 1, { default => 99 } );

  sub sets_options_on_call
       my %p = validate_with
                   ( params => \@_,
                     spec   => { foo => { type SCALAR, default => 2 } },
                     ignore_case   => 1,
                     strip_leading => '-',


The Params::Validate module allows you to validate method or function call parameters to an arbitrary level of specificity. At the simplest level, it is capable of validating the required parameters were given and that no unspecified additional parameters were passed in.

It is also capable of determining that a parameter is of a specific type, that it is an object of a certain class hierarchy, that it possesses certain methods, or applying validation callbacks to arguments.


The module always exports the validate and validate_pos functions.

It also has an additional function available for export, validate_with, which can be used to validate any type of parameters, and set various options on a per-invocation basis.

In addition, it can export the following constants, which are used as part of the type checking. These are SCALAR, ARRAYREF, HASHREF, CODEREF, GLOB, GLOBREF, and SCALARREF, UNDEF, OBJECT, BOOLEAN, and HANDLE. These are explained in the section on Type Validation.

The constants are available via the export tag :types. There is also an :all tag which includes all of the constants as well as the validation_options function.


The validation mechanisms provided by this module can handle both named or positional parameters. For the most part, the same features are available for each. The biggest difference is the way that the validation specification is given to the relevant subroutine. The other difference is in the error messages produced when validation checks fail.

When handling named parameters, the module is capable of handling either a hash or a hash reference transparently.

Subroutines expecting named parameters should call the validate subroutine like this:

 validate( @_, { parameter1 => validation spec,
                 parameter2 => validation spec,
               } );

Subroutines expecting positional parameters should call the validate_pos subroutine like this:

 validate_pos( @_, { validation spec }, { validation spec } );

Mandatory/Optional Parameters

If you just want to specify that some parameters are mandatory and others are optional, this can be done very simply.

For a subroutine expecting named parameters, you would do this:

 validate( @_, { foo => 1, bar => 1, baz => 0 } );

This says that the foo and bar parameters are mandatory and that the baz parameter is optional. The presence of any other parameters will cause an error.

For a subroutine expecting positional parameters, you would do this:

 validate_pos( @_, 1, 1, 0, 0 );

This says that you expect at least 2 and no more than 4 parameters. If you have a subroutine that has a minimum number of parameters but can take any maximum number, you can do this:

 validate_pos( @_, 1, 1, (0) x (@_ - 2) );

This will always be valid as long as at least two parameters are given. A similar construct could be used for the more complex validation parameters described further on.

Please note that this:

 validate_pos( @_, 1, 1, 0, 1, 1 );

makes absolutely no sense, so don't do it. Any zeros must come at the end of the validation specification.

In addition, if you specify that a parameter can have a default, then it is considered optional.

Type Validation

This module supports the following simple types, which can be exported as constants:


    A scalar which is not a reference, such as 10 or 'hello'. A parameter that is undefined is not treated as a scalar. If you want to allow undefined values, you will have to specify SCALAR | UNDEF.


    An array reference such as [1, 2, 3] or \@foo.


    A hash reference such as { a = 1, b => 2 }> or \%bar.


    A subroutine reference such as \&foo_sub or sub { print "hello" }.

  • GLOB

    This one is a bit tricky. A glob would be something like *FOO, but not \*FOO, which is a glob reference. It should be noted that this trick:

     my $fh = do { local *FH; };

    makes $fh a glob, not a glob reference. On the other hand, the return value from Symbol::gensym is a glob reference. Either can be used as a file or directory handle.


    A glob reference such as \*FOO. See the GLOB entry above for more details.


    A reference to a scalar such as \$x.


    An undefined value


    A blessed reference.


    This is a special option, and is just a shortcut for UNDEF | SCALAR.


    This option is also special, and is just a shortcut for GLOB | GLOBREF. However, it seems likely that most people interested in either globs or glob references are likely to really be interested in whether the parameter in questoin could be a valid file or directory handle.

To specify that a parameter must be of a given type when using named parameters, do this:

 validate( @_, { foo => { type => SCALAR },
                 bar => { type => HASHREF } } );

If a parameter can be of more than one type, just use the bitwise or (|) operator to combine them.

 validate( @_, { foo => { type => GLOB | GLOBREF } );

For positional parameters, this can be specified as follows:

 validate_pos( @_, { type => SCALAR | ARRAYREF }, { type => CODEREF } );

Interface Validation

To specify that a parameter is expected to have a certain set of methods, we can do the following:

 validate( @_,
           { foo =>
             # just has to be able to ->bar
             { can => 'bar' } } );

 ... or ...

 validate( @_,
           { foo =>
             # must be able to ->bar and ->print
             { can => [ qw( bar print ) ] } } );

Class Validation

A word of warning. When constructing your external interfaces, it is probably better to specify what methods you expect an object to have rather than what class it should be of (or a child of). This will make your API much more flexible.

With that said, if you want to validate_with that an incoming parameter belongs to a class (or child class) or classes, do:

 validate( @_,
           { foo =>
             { isa => 'My::Frobnicator' } } );

 ... or ...

 validate( @_,
           { foo =>
             { isa => [ qw( My::Frobnicator IO::Handle ) ] } } );
 # must be both, not either!

Regex Validation

If you want to specify that a given parameter must match a specific regular expression, this can be done with "regex" spec key. For example:

 validate( @_,
           { foo =>
             { regex => qr/^\d+$/ } } );

The value of the "regex" key may be either a string or a pre-compiled regex created via qr.

The Regexp::Common module on CPAN is an excellent source of regular expressions suitable for validating input.

Callback Validation

If none of the above are enough, it is possible to pass in one or more callbacks to validate the parameter. The callback will be given the value of the parameter as its sole argument. Callbacks are specified as hash reference. The key is an id for the callback (used in error messages) and the value is a subroutine reference, such as:

 validate( @_,
           { foo =>
             callbacks =>
             { 'smaller than a breadbox' => sub { shift() < $breadbox },
               'green or blue' =>
                sub { $_[0] eq 'green' || $_[0] eq 'blue' } } } );

On a side note, I would highly recommend taking a look at Damian Conway's Regexp::Common module, which could greatly simply the callbacks you use, as it provides patterns useful for validating all sorts of data.

Mandatory/Optional Revisited

If you want to specify something such as type or interface, plus the fact that a parameter can be optional, do this:

 validate( @_, { foo =>
                 { type => SCALAR },
                 bar =>
                 { type => ARRAYREF, optional => 1 } } );

or this for positional parameters:

 validate_pos( @_, { type => SCALAR }, { type => ARRAYREF, optional => 1 } );

By default, parameters are assumed to be mandatory unless specified as optional.

Specifying defaults

If the validate or validate_pos functions are called in a list context, they will return an array or hash containing the original parameters plus defaults as indicated by the validation spec.

If the function is not called in a list context, providing a default in the validation spec still indicates that the parameter is optional.

The hash or array returned from the function will always be a copy of the original parameters, in order to leave @_ untouched for the calling function.

Simple examples of defaults would be:

 my %p = validate( @_, { foo => 1, bar => { default => 99 } } );

 my @p = validate( @_, 1, { default => 99 } );

In scalar context, a hash reference or array reference will be returned, as appropriate.


Validation failure

By default, when validation fails Params::Validate calls Carp::confess. This can be overridden by setting the on_fail option, which is described in the "GLOBAL" OPTIONS section.

Method calls

When using this module to validate the parameters passed to a method call, you will probably want to remove the class/object from the parameter list before calling validate or validate_pos. If your method expects named parameters, then this is necessary for the validate function to actually work, otherwise @_ will not contain a hash, but rather your object (or class) followed by a hash.

Thus the idiomatic usage of validate in a method call will look something like this:

 sub method
     my $self = shift;

     my %params = validate( @_, { foo => 1, bar => { type => ARRAYREF } } );


Because the calling syntax for the validate and validate_pos functions does not make it possible to specify any options other than the the validation spec, it is possible to set some options as pseudo-'globals'. These allow you to specify such things as whether or not the validation of named parameters should be case sensitive, for one example.

These options are called pseudo-'globals' because these settings are only applied to calls originating from the package that set the options.

In other words, if I am in package Foo and I call Params::Validate::validation_options, those options are only in effect when I call validate from package Foo.

While this is quite different from how most other modules operate, I feel that this is necessary in able to make it possible for one module/application to use Params::Validate while still using other modules that also use Params::Validate, perhaps with different options set.

The downside to this is that if you are writing an app with a standard calling style for all functions, and your app has ten modules, each module must include a call to Params::Validate::validation_options.


  • ignore_case => $boolean

    This is only relevant when dealing with named parameters. If it is true, then the validation code will ignore the case of parameter names. Defaults to false.

    When this is turned on, we have to copy more data around internally, leading to a potential speed impact.

  • strip_leading => $characters

    This too is only relevant when dealing with named parameters. If this is given then any parameters starting with these characters will be considered equivalent to parameters without them entirely. For example, if this is specified as '-', then -foo and foo would be considered identical.

    When this is turned on, we have to copy more data around internally, leading to a potential speed impact.

  • allow_extra => $boolean

    If true, then the validation routine will allow extra parameters not named in the validation specification. In the case of positional parameters, this allows an unlimited number of maximum parameters (though a minimum may still be set). Defaults to false.

  • on_fail => $callback

    If given, this callback will be called whenever a validation check fails. It will be called with a single parameter, which will be a string describing the failure. This is useful if you wish to have this module throw exceptions as objects rather than as strings, for example.

    This callback is expected to die internally. If it does not, the validation will proceed onwards, with unpredictable results.

    The default is to simply use the Carp module's confess() function.

  • stack_skip => $number

    This tells Params::Validate how many stack frames to skip when finding a subroutine name to use in error messages. By default, it looks one frame back, at the immediate caller to validate or validate_pos. If this option is set, then the given number of frames are skipped instead.


The validate_with function can be used to set the options listed above on a per-invocation basis. For example:

  my %p =
          ( params => \@_,
            spec   => { foo => { type => SCALAR },
                        bar => { default => 10 } },
            allow_extra => 1,

In addition to the options listed above, it is also possible to set the option called, which should be a string. This string will be used in any error messages caused by a failure to meet the validation spec.

This subroutine will validate named parameters as a hash if the spec parameter is a hash reference. If it is an array reference, the parameters are assumed to be positional.

  my %p =
          ( params => \@_,
            spec   => { foo => { type => SCALAR },
                        bar => { default => 10 } },
            allow_extra => 1,
            called => 'The Quux::Baz class constructor',

  my @p =
          ( params => \@_,
            spec   => [ { type => SCALAR },
                        { default => 10 } ],
            allow_extra => 1,
            called => 'The Quux::Baz class constructor',


If the environment variable PERL_NO_VALIDATION is set to something true, then all calls to the validation functions are turned into no-ops. This may be useful if you only want to use this module during development but don't want the speed hit during production.

The only error that will be caught will be when an odd number of parameters are passed into a function/method that expects a hash.

This environment value is checked only when the module is first loaded. You cannot change it after the module has loaded.


Right now there is no way (short of a callback) to specify that something must be of one of a list of classes, or that it must possess one of a list of methods. If this is desired, it can be added in the future.

Ideally, there would be only one validation function. If someone figures out how to do this, please let me know.


Getargs::Long - similar capabilities with a different interface. If you like what Params::Validate does but not its 'feel' try this one instead.

Carp::Assert and Class::Contract - other modules in the general spirit of validating that certain things are true before/while/after executing actual program code.


Dave Rolsky, <> and Ilya Martynov <>