String::Print - printf alternative


  ### Functional interface

  use String::Print;           # simpelest way
  use String::Print qw/printi printp/, %config;
  printi 'age {years}', years => 12;
  # interpolation of arrays and hashes (serializers)
  printi 'price-list: {prices}', prices => \@p, _join => "+";
  printi 'dump: {c}', c => \%config;
  # same with positional parameters
  printp 'age %d", 12;
  printp 'price-list: %.2f', \@prices;
  printp 'dump: %s', \%settings;
  # modifiers
  printi 'price: {price%.2f}', price => 3.14*VAT*EURO;

  # [0.91] more complex interpolation names
  printi 'filename: {c.filename}', c => \%config;
  printi 'username: {}', user => $user_object;
  printi 'price: {product.price €}', product => $db->product(3);

  ### Object Oriented interface

  use String::Print 'oo';      # import nothing 
  my $f = String::Print->new(%config);
  $f->printi('age {years}', years => 12);
  $f->printp('age %d', 12);
  ### via Log::Report's __* functions (optional translation)

  use Log::Report;             # or Log::Report::Optional
  print __x"age {years}", years => 12;

  ### via Log::Report::Template (Template Toolkit extension)

  [% SET name = 'John Doe' %]
  [% loc("Dear {name},") %]     # includes translation


This module inserts values into (format) strings. It provides printf and sprintf alternatives via both an object oriented and a functional interface.

Read in the "DETAILS" chapter below, why this module provides a better alternative for printf(). Also, some extended examples can be found down there. Take a look at them first, when you start using this module!


The Object Oriented interface

See functions printi(), sprinti(), printp(), and sprintp(): you can also call them as method.

  use String::Print 'oo';
  my $f = String::Print->new(%config);
  $f->printi($format, @params);
  # exactly the same functionality:
  use String::Print 'printi', %config;
  printi $format, @params;

The Object Oriented interface wins when you need the same configuration in multiple source files, or when you need different configurations within one program. In these cases, the hassle of explicitly using the object has some benefits.


 -Option     --Default
  encode_for   undef
  missing_key  <warning>
  modifiers    [ qr/^%\S+/ = \&format_printf]>
  serializers  <useful defaults>
encode_for => HASH|'HTML'

[0.91] The format string and the inserted values will get encoded according to some syntax rules. For instance, encode_entities() of HTML::Entities when you specify the predefined string HTML. See encodeFor().

missing_key => CODE

[0.91] During interpolation, it may be discovered that a key is missing from the parameter list. In that case, a warning is produced and undef inserted. May can overrule that behavior.

modifiers => ARRAY

Add one or more modifier handlers to power of the formatter. They will get preference over the predefined modifiers, but lower than the modifiers passed to print[ip] itself.

serializers => HASH|ARRAY

How to serialize data elements.


  my $f = String::Print->new
    ( modifiers   => [ EUR   => sub {sprintf "%5.2f e", $_[0]} ]
    , serializers => [ UNDEF => sub {'-'} ]
    , encode_for  => 'HTML'

  $f->printi("price: {p EUR}", p => 3.1415); # price: ␣␣3.14 e
  $f->printi("count: {c}", c => undef);      # count: -



The PAIRS are a combination of an selector and a CODE which processes the value when the modifier matches. The selector is a string or (preferred) a regular expression. Later modifiers with the same name overrule earlier definitions. You may also specify an ARRAY of modifiers per print.

See section "Interpolation: Modifiers" about the details.

$obj->encodeFor(HASH|undef|($predefined, %overrule))

[0.91] Enable/define the output encoding. Read section "Output encoding" about the details.


The following are provided as method and as function. You find their explanation further down on this page.

$obj->printi([$fh], $format, PAIRS|HASH);

$obj->printp([$fh], $format, PAIRS|HASH);

$obj->sprinti($format, PAIRS|HASH);

$obj->sprintp($format, LIST, PAIRS);


The functional interface creates a hidden object. You may import any of these functions explicitly, or all together by not specifying the names.

. Example

  use String::Print;           # all
  use String::Print 'sprinti'; # only sprinti

  use String::Print 'printi'   # only printi
    , modifiers   => [ EUR   => sub {sprintf "%5.2f e", $_[0]} ]
    , serializers => [ UNDEF => sub {'-'} ];

  printi "price: {p EUR}", p => 3.1415; # price: ␣␣3.14 e
  printi "count: {c}", c => undef;      # count: -
printi( [$fh], $format, PAIRS|HASH )

Calls sprinti() to fill the data in PAIRS or HASH in $format, and then sends it to the $fh (by default the selected file)

  open my $fh, '>', $file;
  printi $fh, ...

  printi \*STDERR, ...
printp( [$fh], $format, PAIRS|HASH )

Calls sprintp() to fill the data in PAIRS or HASH in $format, and then sends it to the $fh (by default the selected file)

sprinti($format, PAIRS|HASH|OBJECT)

The $format refers to some string, maybe the result of a translation.

The PAIRS (which may be passed as LIST, HASH, or blessed HASH) contains a mixture of special and normal variables to be filled in. The names of the special variables (the options) start with an underscore (_).

 -Option  --Default
  _append   undef
  _count    undef
  _join     ', '
  _prepend  undef
_append => STRING|OBJECT

Text as STRING appended after $format, without interpolation.

_count => INTEGER

Result of the translation process: when Log::Report subroutine __xn is are used for count-sensitive translation. Those function may add more specials to the parameter list.

_join => STRING

Which STRING to use when an ARRAY is being filled-in as parameter.

_prepend => STRING|OBJECT

Text as STRING prepended before $format, without interpolation. This may also be an OBJECT which gets stringified, but variables not filled-in.

sprintp($format, LIST, PAIRS)

Where sprinti() uses named parameters --especially useful when the strings need translation-- this function stays close to the standard sprintf(). All features of POSIX formats are supported. This should say enough: you can use %3$0#5.*d, if you like.

It may be useful to know that the positional $format is rewritten and then fed into sprinti(). Be careful with the length of the LIST: superfluous parameter PAIRS are passed along to sprinti(), and should only contain "specials": parameter names which start with '_'.

example: of the rewrite

  # positional parameters
  my $x = sprintp "dumpfiles: %s\n", \@dumpfiles
     , _join => ':';

  # is rewritten into, and then processed as
  my $x = sprinti "dumpfiles: {_1}\n"
     , _1 => \@dumpfiles, _join => ':';


Why use printi(), not printf()?

The printf() function is provided by Perl's CORE; you do not need to install any module to use it. Why would you use consider using this module?


printf() uses positional parameters, where printi() uses names to refer to the values to be filled-in. Especially in a set-up with translations, where the format strings get extracted into PO-files, it is much clearer to use names. This is also a disadvantage of printp()

pluggable serializers

printi() supports serialization for specific data-types: how to interpolate undef, HASHes, etc.

pluggable modifiers

Especially useful in context of translations, the FORMAT string may contain (language specific) helpers to insert the values correctly.

correct use of utf8

Sized string formatting in printf() is broken: it takes your string as bytes, not Perl strings (which may be utf8). In unicode, one "character" may use many bytes. Also, some characters are displayed double wide, for instance in Chinese. The printi() implementation will use Unicode::GCString for correct behavior.

automatic output encoding (for HTML)

You can globally declare that all produced strings must be encoded in a certain format, for instance that HTML entities should be encoded.

Four components

To fill-in a FORMAT, four clearly separated components play a role:

1. modifiers

How to change the provided values, for instance to hide locale differences.

2. serializer

How to represent (the modified) the values correctly, for instance undef and ARRAYs.

3. conversion

The standard UNIX format rules, like %d. One conversion rule has been added 'S', which provides unicode correct behavior.

4. encoding

Prepare the output for a certain syntax, like HTML.


  # sprinti() replaces "{$key$modifiers$conversion}" by

  # sprintp() replaces "%pos{$modifiers}$conversion" by


  #XXX Your manual-page reader may not support the unicode used
  #XXX in the examples below.
  printi "price: {price € %-10s}", price => $cost;
  printi "price: {price € %-10s}", { price => $cost };
  printp "price: %-10{€}s", $cost;

  $value      = $cost (in €)
  $modifier   = convert € to local currency £
  $serializer = show float as string
  $format     = column width %-10s
  $encode     = £ into &pound;     # when encodingFor('HTML')

Interpolation: keys

A key is a bareword (like a variable name) or a list of barewords separated by dots (no blanks!)

Please use explanatory key names, to help the translation process once you need that (in the future).

Simple keys

A simple key directly refers to a named parameter of the function or method:

  printi "Username: {name}", name => 'John';

You may also pass them as HASH or CODE:

  printi "Username: {name}", { name => 'John' };
  printi "Username: {name}", name => sub { 'John' };
  printi "Username: {name}", { name => sub { 'John' } };
  printi "Username: {name}", name => sub { sub {'John'} };

The smartness of pre-processing CODE is part of serialization.

Complex keys

[0.91] In the previous section, we kept our addressing it simple: let's change that now. Two alternatives for the same:

  my $user = { name => 'John' };
  printi "Username: {name}", name => $user->{name}; # simple key
  printi "Username: {}", user => $user;    # complex key

The way these complex keys work, is close to the flexibility of template toolkit: the only thing you cannot do, is passing parameters to called CODE.

You can pass a parameter name as HASH, which contains values. This may even be nested into multiple levels. You may also pass objects, class (package names), and code references.

In above case of, when user is a HASH it will take the value which belongs to the key name. When user is a CODE, it will run code to get a value. When user is an object, the method name is called to get a value back. When user is a class name, the name refers to an instance method on that class.

More examples which do work:

  # when name is a column in the database query result
  printi "Username: {}", user => $sth->fetchrow_hashref;
  # call a sub which does the database query, returning a HASH
  printi "Username: {}", user => sub { $db->getUser('John') };

  # using an instance method (object)
  { package User;
    sub new  { bless { myname => $_[1] }, $_[0] }
    sub name { $_[0]->{myname} }
  my $user = User->new('John');
  printi "Username: {}", user => $user;

  # using a class method
  sub User::count   { 42 }
  printi "Username: {user.count}", user => 'User';

  # nesting, mixing
  printi "Complain to {product.factory.address}", product => $p;

  # mixed, here CODE, HASH, and Object
  printi "Username: {}", document => sub {
    return +{ author => User->new('John') }

Limitation: you cannot pass arguments to CODE calls.

Interpolation: Serialization

The 'interpolation' functions have named VARIABLES to be filled-in, but also additional OPTIONS. To distinguish between the OPTIONS and VARIABLES (both a list of key-value pairs), the keys of the OPTIONS start with an underscore _. As result of this, please avoid the use of keys which start with an underscore in variable names. On the other hand, you are allowed to interpolate OPTION values in your strings.

There is no way of checking beforehand whether you have provided all values to be interpolated in the translated string. When you refer to value which is missing, it will be interpreted as undef.


Simple scalar values are interpolated "as is"


When a value is passed as CODE reference, that function will get called to return the value to be filled in. For interpolating, the following rules apply:


Takes the value where the scalar reference points to.


All members will be interpolated with ,␣ between the elements. Alternatively (maybe nicer), you can pass an interpolation parameter via the _join OPTION.

  printi "matching files: {files}", files => \@files, _join => ', '

By default, HASHes are interpolated with sorted keys,

   $key => $value, $key2 => $value2, ...

There is no quoting on the keys or values (yet). Usually, this will produce an ugly result anyway.


With the serialization parameter, you can overrule the interpolation of above defaults, but also add rules for your own objects. By default, objects get stringified.

  serialization => [ $myclass => \&name_in_reverse ]

  sub name_in_reverse($$$)
  {   my ($formatter, $object, $args) = @_;
      # the $args are all parameters to be filled-in
      scalar reverse $object->name;

Interpolation: Modifiers

Modifiers are used to change the value to be inserted, before the characters get interpolated in the line. This is a powerful simplification. Let's discuss this with an example.

In traditional (gnu) gettext, you would write:

  printf(gettext("approx pi: %.6f\n"), PI);

to get PI printed with six digits in the fragment. Locale::TextDomain has two ways to achieve that:

  printf __"approx pi: %.6f\n", PI;
  print __x"approx pi: {approx}\n", approx => sprintf("%.6f", PI);

The first does not respect the wish to be able to reorder the arguments during translation (although there are ways to work around that) The second version is quite long. The string to be translated differs between the two examples.

With Log::Report, above syntaxes do work as well, but you can also do:

  # with optional translations
  print __x"approx pi: {pi%.6f}\n", pi => PI;

The base for __x() is the printi() provided by this module. Internally, it will call printi to fill-in parameters:

  printi "approx pi: {pi%.6f}\n", pi => PI;

Another example:

  printi "{perms} {links%2d} {user%-8s} {size%10d} {fn}\n",
     perms => '-rw-r--r--', links => 7, user => 'me',
     size => 12345, fn => $filename;

An additional advantage (when you use translation) is the fact that not all languages produce comparable length strings. Now, the translators can change the format, such that the layout of tables is optimal for their language.

Above example in printp() syntax, shorter but less maintainable:

  printp "%s %2d %-8s 10d %s\n",
     '-rw-r--r--', 7, 'me', 12345, $filename;

Interpolation: default modifiers

Default modifier: POSIX format

As shown in the examples above, you can specify a format. This can, for instance, help you with rounding or columns:

  printp "π = {pi%.3f}", pi => 3.1415;
  printp "weight is {kilogram%d}", kilogram => 127*OUNCE_PER_KILO;
  printp "{filename%-20.20s}\n", filename => $fn;

- improvements on POSIX format

The POSIX printf() does not handle unicode strings. Perl does understand that the 's' modifier may need to insert utf8 so does not count bytes but characters. printi() does not use characters but "grapheme clusters" via Unicode::GCString. Now, also composed characters do work correctly.

Additionally, you can use the new 'S' conversion to count in columns. In fixed-width fonts, graphemes can have width 0, 1 or 2. For instance, Chinese characters have width 2. When printing in fixed-width, this 'S' is probably the better choice over 's'. When the field does not specify its width, then there is no performance penalty for using 'S'.

  # name right aligned, commas on same position, always
  printp "name: {name%20S},\n", name => $some_chinese;

Default modifier: BYTES

[0.91] Too often, you have to translate a (file) size into humanly readible format. The BYTES modifier simplifies this a lot:

  printp "{size BYTES} {fn}\n", fn => $fn, size => -s $fn;

The output will always be 6 characters. Examples are "999 B", "1.2 kB", and " 27 MB".

Default modifiers: YEAR, DATE, TIME, DT, and DT()

[0.91] A set of modifiers help displaying dates and times. They are a little flexible in values they accept, but do not expect miracles: when it get harder, you will need to process it yourself.

The actual treatment of a time value depends on the value: three different situations:

1. numeric

A pure numeric value is considered "seconds since epoch", unless it is smaller than 21000000, in which case it is taken as date without separators.

2. date format without time-zone

The same formats are understood as in the next option, but without time-zone information. The date is processed as text as if in the local time zone, and the output in the local time-zone.

3. date format with time-zone

By far not all possible date formats are supported, just a few common versions, like

  2017-06-27 10:04:15 +02:00
  2017-06-27 17:34:28.571491+02  # psql timestamp with zone
  2017-06-27T10:04:15Z           # iso 8601
  20170627                       # only for YEAR and DATE
  2017-6-1                       # only for YEAR and DATE
  12:34                          # only for TIME

The meaning of 05-04-2017 is unclear, so not supported. Milliseconds get ignored.

When the provided value has a timezone indication, it will get converted into the local timezone of the observer.

The output of YEAR is in format 'YYYY', for DATE it will always be 'YYYY-MM-DD', where TIME produces 'HH:mm:ss'.

The short form DT is an alias for DT(FT). The DT modifier can produce different formats:

  DT(ASC)     : %a %b %e %T %Y       asctime output
  DT(FT)      : %F %T                YYYY-MM-DD HH:mm:ss
  DT(ISO)     : %FT%T%z              iso8601
  DT(RFC822)  : %a, %d %b %y %T %z   email old
  DT(RFC2822) : %a, %d %b %Y %T %z   email newer

You may suggest additional formats, or add your own modifier.

Default modifiers: //word, //"string", //'string'

[0.91] By default, an undefined value is shown as text 'undef'. Empty strings are shown as nothing. This may not be nice. You may want to be more specific when a value is missing.

   "visitors: {count //0}"
   "published: {date DT//'not yet'}"
   "copyright: {year//2017 YEAR}

Modifiers will usually return undef when they are called with an undefined or empty value. By the right order of '//', you may product different kinds of output:

   "price: {price//5 EUR}"
   "price: {price EUR//unknown}"

Private modifiers

You may pass your own modifiers. A modifier consists of a selector and a CODE, which is called when the selector matches. The selector is either a string or a regular expression.

  # in Object Oriented syntax:
  my $f = String::Print->new
    ( modifiers => [ qr/[€₤]/ => \&money ]

  # in function syntax:
  use String::Print 'printi', 'sprinti'
    , modifiers => [ qr/[€₤]/ => \&money ];

  # the implementation:
  sub money$$$$)
  { my ($formatter, $modif, $value, $args) = @_;

      $modif eq '€' ? sprintf("%.2f EUR", $value+0.0001)
    : $modif eq '₤' ? sprintf("%.2f GBP", $value/1.16+0.0001)
    :                 'ERROR';

Using printp() makes it a little shorter, but will become quite complex when there are more parameter in one string.

  printi "price: {p€}", p => $pi;   # price: 3.14 EUR
  printi "price: {p₤}", p => $pi;   # price: 2.71 GBP

  printp "price: %{€}s", $pi;       # price: 3.14 EUR
  printp "price: %{₤}s", $pi;       # price: 2.71 GBP

This is very useful in the translation context, where the translator can specify abstract formatting rules. As example, see the (GNU) gettext files, in the translation table for Dutch into English. The translator tells us which currency to use in the display.

  msgid  "kostprijs: {p€}"
  msgstr "price: {p₤}"

Another example. Now, we want to add timestamps. In this case, we decide for modifier names in \w, so we need a blank to separate the parameter from the modifer.

Modifiers: stacking

You can add more than one modifier. The modifiers detect the extend of their own information (via a regular expression), and therefore the formatter understands where one ends and the next begins.

The modifiers are called in order:

  printi "price: {p€%9s}\n", p => $p; # price: ␣␣␣123.45
  printi ">{t T%10s}<", t => $now;    # >␣␣12:59:17<

  printp "price: %9{€}s\n", $p;       # price: ␣␣␣123.45
  printp ">%10{T}s<", $now;           # >␣␣12:59:17<

Output encoding

[0.91] This module is also used by Log::Report::Template, which is used to insert (translated) strings with parameters into HTML templates. You can imagine that some of the parameter may need to be encoded to HTML in the template, and other not.

example with Log::Report::Template

In pure Template Toolkit, you would write

  # in your TT-template
  <div>Username: [% username | html %]</div>
  # in your code
  username => $user->name,

With plain String::Print with output encoding enabled, you can do:

  # in your TT-template
  <div>[% show_username %]</div>
  # in your code with encodeFor('HTML')
  show_username => printi("Username: {user}", user => $user->name),
  # or
  show_username => printp("Username: %s", $user->name),

That does not look very efficient, however it changes for the good when this is combined with Log::Report::Lexicon (translations) You can either do:

  # in your TT-template
  <div>[% show_username %]</div>
  # in your code with encodeFor('HTML')
  show_username => __x("Username: {user}", user => $user->name),


  # in your TT-template with encodeFor('HTML')
  <div>[% loc("Username: {user}", user => username) %]</div>
  # in your code
  username => $user->name,

Even shorter:

  # in your TT-template with encodeFor('HTML')
  <div>[% loc("Username: {}", user => userobj) %]</div>
  # in your code
  userobj => $user,


  # in your TT-template with encodeFor('HTML')
  <div>[% loc("Username: {}") %]</div>
  # in your code
  user => $user,

Shorter that the original, and translations for free! More examples in Log::Report::Template.

Output encoding exclusion

In some cases, the data which is inserted is already encoded in the output syntax. For instance, you already have HTML to be included.

The default exclusion rule for HTML output is qr/html$/i, which means that all inserted named parameters, where the name ends on html will not get html-entity encoded.

This will work by default:

  # with encodeFor('HTML')
  printp "Me & Co: {name}, {description_html}",
     name => 'René', description_html => $descr;

This may result in:

  Me &amp; Co: Ren&eacute;, <font color="red">new member</font>

Better not to have HTML in your program: leave it to the template. But in some cases, you have no choice.

Compared to other modules on CPAN

There are a quite a number of modules on CPAN which extend the functionality of printf(). To name a few: String::Format, String::Errf, String::Formatter, Text::Sprintf::Named, Acme::StringFormat, Text::sprintf, Log::Sprintf, and String::Sprintf. They are all slightly different.

When the String::Print module was created, none of the modules mentioned above handled unicode correctly. Global configuration of serializers and modifiers is also usually not possible, sometimes provided per explicit function call. Only String::Print cleanly separates the roles of serializers, modifiers, and conversions.

String::Print is nicely integrated with Log::Report.


This module is part of String-Print distribution version 0.94, built on March 01, 2020. Website:


Copyrights 2016-2020 by [Mark Overmeer <>]. For other contributors see ChangeLog.

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