Params::Check - A generic input parsing/checking mechanism.


use Params::Check qw[check allow last_error];

sub fill_personal_info {
    my %hash = @_;
    my $x;

    my $tmpl = {
        firstname   => { required   => 1, defined => 1 },
        lastname    => { required   => 1, store => \$x },
        gender      => { required   => 1,
                         allow      => [qr/M/i, qr/F/i],
        married     => { allow      => [0,1] },
        age         => { default    => 21,
                         allow      => qr/^\d+$/,

        phone       => { allow => [ sub { return 1 if /$valid_re/ },
                                    '1-800-PERL' ]
        id_list     => { default        => [],
                         strict_type    => 1
        employer    => { default => 'NSA', no_override => 1 },

    ### check() returns a hashref of parsed args on success ###
    my $parsed_args = check( $tmpl, \%hash, $VERBOSE )
                        or die qw[Could not parse arguments!];

    ... other code here ...

my $ok = allow( $colour, [qw|blue green yellow|] );

my $error = Params::Check::last_error();


Params::Check is a generic input parsing/checking mechanism.

It allows you to validate input via a template. The only requirement is that the arguments must be named.

Params::Check can do the following things for you:

Most of Params::Check's power comes from its template, which we'll discuss below:


As you can see in the synopsis, based on your template, the arguments provided will be validated.

The template can take a different set of rules per key that is used.

The following rules are available:


This is the default value if none was provided by the user. This is also the type strict_type will look at when checking type integrity (see below).


A boolean flag that indicates if this argument was a required argument. If marked as required and not provided, check() will fail.


This does a ref() check on the argument provided. The ref of the argument must be the same as the ref of the default value for this check to pass.

This is very useful if you insist on taking an array reference as argument for example.


If this template key is true, enforces that if this key is provided by user input, its value is defined. This just means that the user is not allowed to pass undef as a value for this key and is equivalent to: allow => sub { defined $_[0] && OTHER TESTS }


This allows you to specify constants in your template. ie, they keys that are not allowed to be altered by the user. It pretty much allows you to keep all your configurable data in one place; the Params::Check template.


This allows you to pass a reference to a scalar, in which the data will be stored:

my $x;
my $args = check(foo => { default => 1, store => \$x }, $input);

This is basically shorthand for saying:

my $args = check( { foo => { default => 1 }, $input );
my $x    = $args->{foo};

You can alter the global variable $Params::Check::NO_DUPLICATES to control whether the store'd key will still be present in your result set. See the "Global Variables" section below.


A set of criteria used to validate a particular piece of data if it has to adhere to particular rules.

See the allow() function for details.


check( \%tmpl, \%args, [$verbose] );

This function is not exported by default, so you'll have to ask for it via:

use Params::Check qw[check];

or use its fully qualified name instead.

check takes a list of arguments, as follows:


This is a hash reference which contains a template as explained in the SYNOPSIS and Template section.


This is a reference to a hash of named arguments which need checking.


A boolean to indicate whether check should be verbose and warn about what went wrong in a check or not.

You can enable this program wide by setting the package variable $Params::Check::VERBOSE to a true value. For details, see the section on Global Variables below.

check will return when it fails, or a hashref with lowercase keys of parsed arguments when it succeeds.

So a typical call to check would look like this:

my $parsed = check( \%template, \%arguments, $VERBOSE )
                or warn q[Arguments could not be parsed!];

A lot of the behaviour of check() can be altered by setting package variables. See the section on Global Variables for details on this.

allow( $test_me, \@criteria );

The function that handles the allow key in the template is also available for independent use.

The function takes as first argument a key to test against, and as second argument any form of criteria that are also allowed by the allow key in the template.

You can use the following types of values for allow:


The provided argument MUST be equal to the string for the validation to pass.


The provided argument MUST match the regular expression for the validation to pass.


The provided subroutine MUST return true in order for the validation to pass and the argument accepted.

(This is particularly useful for more complicated data).

array ref

The provided argument MUST equal one of the elements of the array ref for the validation to pass. An array ref can hold all the above values.

It returns true if the key matched the criteria, or false otherwise.


Returns a string containing all warnings and errors reported during the last time check was called.

This is useful if you want to report then some other way than carp'ing when the verbose flag is on.

It is exported upon request.

Global Variables

The behaviour of Params::Check can be altered by changing the following global variables:


This controls whether Params::Check will issue warnings and explanations as to why certain things may have failed. If you set it to 0, Params::Check will not output any warnings.

The default is 1 when warnings are enabled, 0 otherwise;


This works like the strict_type option you can pass to check, which will turn on strict_type globally for all calls to check.

The default is 0;


If you set this flag, unknown options will still be present in the return value, rather than filtered out. This is useful if your subroutine is only interested in a few arguments, and wants to pass the rest on blindly to perhaps another subroutine.

The default is 0;


If you set this flag, all keys passed in the following manner:

function( -key => 'val' );

will have their leading dashes stripped.


If set to true, all keys in the template that are marked as to be stored in a scalar, will also be removed from the result set.

Default is false, meaning that when you use store as a template key, check will put it both in the scalar you supplied, as well as in the hashref it returns.


If set to true, Params::Check will no longer convert all keys from the user input to lowercase, but instead expect them to be in the case the template provided. This is useful when you want to use similar keys with different casing in your templates.

Understand that this removes the case-insensitivity feature of this module.

Default is 0;


If set to true, Params::Check will require all values passed to be defined. If you wish to enable this on a 'per key' basis, use the template option defined instead.

Default is 0;


If set to true, Params::Check will sanity check templates, validating for errors and unknown keys. Although very useful for debugging, this can be somewhat slow in hot-code and large loops.

To disable this check, set this variable to false.

Default is 1;


If set to true, Params::Check will croak when an error during template validation occurs, rather than return false.

Default is 0;


This global modifies the argument given to caller() by Params::Check::check() and is useful if you have a custom wrapper function around Params::Check::check(). The value must be an integer, indicating the number of wrapper functions inserted between the real function call and Params::Check::check().

Example wrapper function, using a custom stacktrace:

sub check {
    my ($template, $args_in) = @_;

    local $Params::Check::WARNINGS_FATAL = 1;
    local $Params::Check::CALLER_DEPTH = $Params::Check::CALLER_DEPTH + 1;
    my $args_out = Params::Check::check($template, $args_in);

    my_stacktrace(Params::Check::last_error) unless $args_out;

    return $args_out;

Default is 0;


Thanks to Richard Soderberg for his performance improvements.


Please report bugs or other issues to <>.


This module by Jos Boumans <>.


This library is free software; you may redistribute and/or modify it under the same terms as Perl itself.