Scroll to navigation

DateTime::Format::Builder(3pm) User Contributed Perl Documentation DateTime::Format::Builder(3pm)


DateTime::Format::Builder - Create DateTime parser classes and objects.


version 0.83


    package DateTime::Format::Brief;
    use DateTime::Format::Builder (
        parsers => {
            parse_datetime => [
                    regex  => qr/^(\d{4})(\d\d)(\d\d)(\d\d)(\d\d)(\d\d)$/,
                    params => [qw( year month day hour minute second )],
                    regex  => qr/^(\d{4})(\d\d)(\d\d)$/,
                    params => [qw( year month day )],


DateTime::Format::Builder creates DateTime parsers. Many string formats of dates and times are simple and just require a basic regular expression to extract the relevant information. Builder provides a simple way to do this without writing reams of structural code.

Builder provides a number of methods, most of which you'll never need, or at least rarely need. They're provided more for exposing of the module's innards to any subclasses, or for when you need to do something slightly beyond what I expected.


See DateTime::Format::Builder::Tutorial.


Often, I will speak of "undef" being returned, however that's not strictly true.

When a simple single specification is given for a method, the method isn't given a single parser directly. It's given a wrapper that will call "on_fail" if the single parser returns "undef". The single parser must return "undef" so that a multiple parser can work nicely and actual errors can be thrown from any of the callbacks.

Similarly, any multiple parsers will only call "on_fail" right at the end when it's tried all it could.

"on_fail" (see later) is defined, by default, to throw an error.

Multiple parser specifications can also specify "on_fail" with a coderef as an argument in the options block. This will take precedence over the inheritable and overrideable method.

That said, don't throw real errors from callbacks in multiple parser specifications unless you really want parsing to stop right there and not try any other parsers.

In summary: calling a method will result in either a "DateTime" object being returned or an error being thrown (unless you've overridden "on_fail" or "create_method", or you've specified a "on_fail" key to a multiple parser specification).

Individual parsers (be they multiple parsers or single parsers) will return either the "DateTime" object or "undef".


A single specification is a hash ref of instructions on how to create a parser.

The precise set of keys and values varies according to parser type. There are some common ones though:

  • length

    length is an optional parameter that can be used to specify that this particular regex is only applicable to strings of a certain fixed length. This can be used to make parsers more efficient. It's strongly recommended that any parser that can use this parameter does.

    You may happily specify the same length twice. The parsers will be tried in order of specification.

    You can also specify multiple lengths by giving it an arrayref of numbers rather than just a single scalar. If doing so, please keep the number of lengths to a minimum.

    If any specifications without lengths are given and the particular length parser fails, then the non-length parsers are tried.

    This parameter is ignored unless the specification is part of a multiple parser specification.

  • label

    label provides a name for the specification and is passed to some of the callbacks about to mentioned.

  • on_match and on_fail

    on_match and on_fail are callbacks. Both routines will be called with parameters of:

  • input

    input is the input to the parser (after any preprocessing callbacks).

  • label

    label is the label of the parser if there is one.

  • self

    self is the object on which the method has been invoked (which may just be a class name). Naturally, you can then invoke your own methods on it do get information you want.

  • args is an arrayref of any passed arguments, if any. If there were no arguments, then this parameter is not given.

These routines will be called depending on whether the regex match succeeded or failed.


preprocess is a callback provided for cleaning up input prior to parsing. It's given a hash as arguments with the following keys:

  • input

    input is the datetime string the parser was given (if using multiple specifications and an overall preprocess then this is the date after it's been through that preprocessor).

  • parsed

    parsed is the state of parsing so far. Usually empty at this point unless an overall preprocess was given. Items may be placed in it and will be given to any postprocessor and "DateTime->new" (unless the postprocessor deletes it).

  • self, args, label

    self, args, label as per on_match and on_fail.

The return value from the routine is what is given to the regex. Note that this is last code stop before the match.

Note: mixing length and a preprocess that modifies the length of the input string is probably not what you meant to do. You probably meant to use the multiple parser variant of preprocess which is done before any length calculations. This "single parser" variant of preprocess is performed after any length calculations.


postprocess is the last code stop before "DateTime->new" is called. It's given the same arguments as preprocess. This allows it to modify the parsed parameters after the parse and before the creation of the object. For example, you might use:

        regex       => qr/^(\d\d) (\d\d) (\d\d)$/,
        params      => [qw( year  month  day   )],
        postprocess => \&_fix_year,

where "_fix_year" is defined as:

    sub _fix_year {
        my %args = @_;
        my ( $date, $p ) = @args{qw( input parsed )};
        $p->{year} += $p->{year} > 69 ? 1900 : 2000;
        return 1;

This will cause the two digit years to be corrected according to the cut off. If the year was '69' or lower, then it is made into 2069 (or 2045, or whatever the year was parsed as). Otherwise it is assumed to be 19xx. The DateTime::Format::Mail module uses code similar to this (only it allows the cut off to be configured and it doesn't use Builder).

Note: It is very important to return an explicit value from the postprocess callback. If the return value is false then the parse is taken to have failed. If the return value is true, then the parse is taken to have succeeded and "DateTime->new" is called.

See the documentation for the individual parsers for their valid keys.

Parsers at the time of writing are:

  • DateTime::Format::Builder::Parser::Regex - provides regular expression based parsing.
  • DateTime::Format::Builder::Parser::Strptime - provides strptime based parsing.

Subroutines / coderefs as specifications.

A single parser specification can be a coderef. This was added mostly because it could be and because I knew someone, somewhere, would want to use it.

If the specification is a reference to a piece of code, be it a subroutine, anonymous, or whatever, then it's passed more or less straight through. The code should return "undef" in event of failure (or any false value, but "undef" is strongly preferred), or a true value in the event of success (ideally a "DateTime" object or some object that has the same interface).

This all said, I generally wouldn't recommend using this feature unless you have to.


I mention a number of callbacks in this document.

Any time you see a callback being mentioned, you can, if you like, substitute an arrayref of coderefs rather than having the straight coderef.


These are very easily described as an array of single specifications.

Note that if the first element of the array is an arrayref, then you're specifying options.

  • preprocess

    preprocess lets you specify a preprocessor that is called before any of the parsers are tried. This lets you do things like strip off timezones or any unnecessary data. The most common use people have for it at present is to get the input date to a particular length so that the length is usable (DateTime::Format::ICal would use it to strip off the variable length timezone).

    Arguments are as for the single parser preprocess variant with the exception that label is never given.

  • on_fail

    on_fail should be a reference to a subroutine that is called if the parser fails. If this is not provided, the default action is to call "DateTime::Format::Builder::on_fail", or the "on_fail" method of the subclass of DTFB that was used to create the parser.


Builder allows you to plug in a fair few callbacks, which can make following how a parse failed (or succeeded unexpectedly) somewhat tricky.

For Single Specifications

A single specification will do the following:

User calls parser:

    my $dt = $class->parse_datetime($string);
preprocess is called. It's given $string and a reference to the parsing workspace hash, which we'll call $p. At this point, $p is empty. The return value is used as $date for the rest of this single parser. Anything put in $p is also used for the rest of this single parser.
regex is applied.
If regex did not match, then on_fail is called (and is given $date and also label if it was defined). Any return value is ignored and the next thing is for the single parser to return "undef".

If regex did match, then on_match is called with the same arguments as would be given to on_fail. The return value is similarly ignored, but we then move to step 4 rather than exiting the parser.

postprocess is called with $date and a filled out $p. The return value is taken as a indication of whether the parse was a success or not. If it wasn't a success then the single parser will exit at this point, returning undef.
"DateTime->new" is called and the user is given the resultant "DateTime" object.

See the section on error handling regarding the "undef"s mentioned above.

For Multiple Specifications

With multiple specifications:

User calls parser:

    my $dt = $class->complex_parse($string);
The overall preprocessor is called and is given $string and the hashref $p (identically to the per parser preprocess mentioned in the previous flow).

If the callback modifies $p then a copy of $p is given to each of the individual parsers. This is so parsers won't accidentally pollute each other's workspace.

If an appropriate length specific parser is found, then it is called and the single parser flow (see the previous section) is followed, and the parser is given a copy of $p and the return value of the overall preprocessor as $date.

If a "DateTime" object was returned so we go straight back to the user.

If no appropriate parser was found, or the parser returned "undef", then we progress to step 3!

Any non-length based parsers are tried in the order they were specified.

For each of those the single specification flow above is performed, and is given a copy of the output from the overall preprocessor.

If a real "DateTime" object is returned then we exit back to the user.

If no parser could parse, then an error is thrown.

See the section on error handling regarding the "undef"s mentioned above.


In the general course of things you won't need any of the methods. Life often throws unexpected things at us so the methods are all available for use.


"import" is a wrapper for "create_class". If you specify the class option (see documentation for "create_class") it will be ignored.


This method can be used as the runtime equivalent of "import". That is, it takes the exact same parameters as when one does:

    use DateTime::Format::Builder ( ... )

That can be (almost) equivalently written as:

    use DateTime::Format::Builder;
    DateTime::Format::Builder->create_class( ... );

The difference being that the first is done at compile time while the second is done at run time.

In the tutorial I said there were only two parameters at present. I lied. There are actually three of them.

  • parsers

    parsers takes a hashref of methods and their parser specifications. See the DateTime::Format::Builder::Tutorial for details.

    Note that if you define a subroutine of the same name as one of the methods you define here, an error will be thrown.

  • constructor

    constructor determines whether and how to create a "new" function in the new class. If given a true value, a constructor is created. If given a false value, one isn't.

    If given an anonymous sub or a reference to a sub then that is used as "new".

    The default is 1 (that is, create a constructor using our default code which simply creates a hashref and blesses it).

    If your class defines its own "new" method it will not be overwritten. If you define your own "new" and also tell Builder to define one an error will be thrown.

  • verbose

    verbose takes a value. If the value is "undef", then logging is disabled. If the value is a filehandle then that's where logging will go. If it's a true value, then output will go to "STDERR".

    Alternatively, call $DateTime::Format::Builder::verbose with the relevant value. Whichever value is given more recently is adhered to.

    Be aware that verbosity is a global setting.

  • class

    class is optional and specifies the name of the class in which to create the specified methods.

    If using this method in the guise of "import" then this field will cause an error so it is only of use when calling as "create_class".

  • version

    version is also optional and specifies the value to give $VERSION in the class. It's generally not recommended unless you're combining with the class option. A "ExtUtils::MakeMaker" / "CPAN" compliant version specification is much better.

In addition to creating any of the methods it also creates a "new" method that can instantiate (or clone) objects.


In the rest of the documentation I've often lied in order to get some of the ideas across more easily. The thing is, this module's very flexible. You can get markedly different behaviour from simply subclassing it and overriding some methods.


Given a parser coderef, returns a coderef that is suitable to be a method.

The default action is to call "on_fail" in the event of a non-parse, but you can make it do whatever you want.


This is called in the event of a non-parse (unless you've overridden "create_method" to do something else.

The single argument is the input string. The default action is to call "croak". Above, where I've said parsers or methods throw errors, this is the method that is doing the error throwing.

You could conceivably override this method to, say, return "undef".


The methods listed in the METHODS section are all you generally need when creating your own class. Sometimes you may not want a full blown class to parse something just for this one program. Some methods are provided to make that task easier.


The basic constructor. It takes no arguments, merely returns a new "DateTime::Format::Builder" object.

    my $parser = DateTime::Format::Builder->new;

If called as a method on an object (rather than as a class method), then it clones the object.

    my $clone = $parser->new;


Provided for those who prefer an explicit "clone" method rather than using "new" as an object method.

    my $clone_of_clone = $clone->clone;


Given either a single or multiple parser specification, sets the object to have a parser based on that specification.

        regex  => qr/^ (\d{4}) (\d\d) (\d\d) $/x;
        params => [qw( year    month  day    )],

The arguments given to "parser" are handed directly to "create_parser". The resultant parser is passed to "set_parser".

If called as an object method, it returns the object.

If called as a class method, it creates a new object, sets its parser and returns that object.


Sets the parser of the object to the given parser.


Note: this method does not take specifications. It also does not take anything except coderefs. Luckily, coderefs are what most of the other methods produce.

The method return value is the object itself.


Returns the parser the object is using.

    my $code = $parser->get_parser;


Given a string, it calls the parser and returns the "DateTime" object that results.

    my $dt = $parser->parse_datetime('1979 07 16');

The return value, if not a "DateTime" object, is whatever the parser wants to return. Generally this means that if the parse failed an error will be thrown.


If you call this function, it will throw an error.


Some longer examples are provided in the distribution. These implement some of the common parsing DateTime modules using Builder. Each of them are, or were, drop in replacements for the modules at the time of writing them.


Dave Rolsky (DROLSKY) for kickstarting the DateTime project, writing DateTime::Format::ICal and DateTime::Format::MySQL, and some much needed review.

Joshua Hoblitt (JHOBLITT) for the concept, some of the API, impetus for writing the multi-length code (both one length with multiple parsers and single parser with multiple lengths), blame for the Regex custom constructor code, spotting a bug in Dispatch, and more much needed review.

Kellan Elliott-McCrea (KELLAN) for even more review, suggestions, DateTime::Format::W3CDTF and the encouragement to rewrite these docs almost 100%!

Claus Färber (CFAERBER) for having me get around to fixing the auto-constructor writing, providing the 'args'/'self' patch, and suggesting the multi-callbacks.

Rick Measham (RICKM) for DateTime::Format::Strptime which Builder now supports.

Matthew McGillis for pointing out that "on_fail" overriding should be simpler.

Simon Cozens (SIMON) for saying it was cool.


"" mailing list.

perl, DateTime, DateTime::Format::Builder::Tutorial, DateTime::Format::Builder::Parser


Bugs may be submitted at <>.

I am also usually active on IRC as 'autarch' on "irc://".


The source code repository for DateTime-Format-Builder can be found at <>.


If you'd like to thank me for the work I've done on this module, please consider making a "donation" to me via PayPal. I spend a lot of free time creating free software, and would appreciate any support you'd care to offer.

Please note that I am not suggesting that you must do this in order for me to continue working on this particular software. I will continue to do so, inasmuch as I have in the past, for as long as it interests me.

Similarly, a donation made in this way will probably not make me work on this software much more, unless I get so many donations that I can consider working on free software full time (let's all have a chuckle at that together).

To donate, log into PayPal and send money to, or use the button at <>.


  • Dave Rolsky <>
  • Iain Truskett <>


  • Daisuke Maki <>
  • James Raspass <>


This software is Copyright (c) 2020 by Dave Rolsky.

This is free software, licensed under:

  The Artistic License 2.0 (GPL Compatible)

The full text of the license can be found in the LICENSE file included with this distribution.

2020-08-11 perl v5.30.3