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Manual Reference Pages  -  CONFIG::WREST (3)

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Config::Wrest - Read and write Configuration data With References, Environment variables, Sections, and Templating



        use Config::Wrest;
        my $c = new Config::Wrest();

        # Read configuration data from a string, or from a reference to a string
        my $vars;
        $vars = $c->deserialize($string);
        $vars = $c->deserialize(\$string);

        # Write configuration data as a string
        my $string = $c->serialize(\%vars);
        # ...write the data into a specific scalar
        $c->serialize(\%vars, \$string);

        # Convenience methods to interface with files
        $vars = $c->parse_file($filename);
        $c->write_file($filename, \%vars);


This module allows you to read configuration data written in a human-readable and easily-editable text format and access it as a perl data structure. It also allows you to write configuration data from perl back to this format.

The data format allows key/value pairs, comments, escaping of unprintable or problematic characters, sensible whitespace handling, support for Unicode data, nested sections, or blocks, of configuration data (analogous to hash- and array-references), and the optional preprocessing of each line through a templating engine. If you choose to use a templating engine then, depending on the engine you’re using, you can interpolate other values into the data, interpolate environment variables, and perform other logic or transformations. The data format also allows you to use directives to alter the behaviour of the parser from inside the configuration file, to set variables, to include other files, and for other actions.

Here’s a brief example of some configuration data. Note the use of quotes, escape sequences, and nested blocks:

        Language =  perl
                width = 100     # This is an end-of-line comment
                height  100
                alt_text " square red image, copyright %A9 2001 "
                        colour red
        @include path/to/file.cfg
                Full Moon
                        length 48h
                # and so on... This is a full-line comment

This parses to the perl data structure:

                Language => perl,
                imageinfo => {
                        width => 100,
                        height => 100,
                        alt_text => " square red image, copyright \xA9 2001 ",
                        Nestedblock => {
                                colour => red
                        Suffixes => [
                Days => [
                        "Can\x{2019}t", # note the Unicode character in this string
                        Full Moon,
                                length => 48h
                # ...and of course, whatever data was read from the included file "path/to/file.cfg"

Of course, your configuration data may not need to use any of those special features, and might simply be key/value pairs:

        Basedir   /usr/local/myprogram
        Debug     0
        Database  IFL1

This parses to the perl data structure:

                Basedir => /usr/local/myprogram,
                Debug => 0,
                Database => IFL1,

These data structures can be serialized back to a textual form using this module.

For details of the data format see DATA FORMAT and DIRECTIVES. Also see CONSTRUCTOR OPTIONS for options which affect the parsing of the data. All file input and output goes through File::Slurp::WithinPolicy.


Although the Wrest in the module’s name is an abbreviation for its main features, it also means a key to tune a stringed instrument or active or moving power. (Collaborative International Dictionary of English) You can also think of it wresting your configuration data from human-readable form into perl.


new( %OPTIONS ) Return a new object, configured with the given options - see CONSTRUCTOR OPTIONS.
deserialize( $STRING ) or deserialize( \$STRING ) Given either a string containing configuration data, or a reference to such a string, attempts to parse it and returns the configuration information as a hash reference. See READING DATA for details of warnings and errors.
serialize( \%VARS ) or serialize( \%VARS, \$STRING ) Given a reference to a hash of configuration data, turns it back into its textual representation. If no string reference is supplied then this text string is returned, otherwise it is written into the given reference. See WRITING DATA for details of warnings and errors.
deserialise() An alias for deserialize()
serialise() An alias for serialize()
parse_file( $FILEPATH ) Read the specified file, deserialize the contents and return the configuration data.
write_file( $FILEPATH, \%VARS ) Serializes the given configuration data and writes it to the specified file.


These are the options that can be supplied to the constructor, and some may meaningfully be modified by the @option directive - namely the UseQuotes, Escapes, Subs and TemplateBackend options. Some of these option are turned on by default.
AllowEmptyValues Default is 1. In this configuration data, one of the keys - Wings - has no value against it:

        Species cod
        Category fish

By default this will be interpreted as the empty string. If this option is set to false then the line will be skipped. A warning will also be emitted unless the IgnoreInvalidLines option is true.

This option also affects the serialization of data. When it’s true it will also allow the serializer to create a configuration line like the Wings example, i.e. a key with an empty value, and allow serialization of empty values in arrays. However, if AllowEmptyValues was false then the serializer would see that the value for Wings was empty and would skip over it, emitting a warning by default. See the ’IgnoreInvalidLines’ option for a way to suppress these warnings.

If you want to read an empty value in a list it needs to be quoted (see the UseQuotes option) otherwise it’ll look like a completely blank line:


Similarly, the UseQuotes option should be in effect if you wish to write out empty values in list blocks, so that they do not appear as blank lines.

DieOnNonExistantVars Default is 1. Usually the parser will die() if the configuration data references a variable that has not been previously declared. However, setting this option to 0 will disable this behaviour and silently continue parsing.
Escapes Default is 1. Translates escape sequences of the form ’%[0-9a-fA-F][0-9a-fA-F]’ or ’%{[0-9a-fA-F]+}’into the character represented by the given hex number. E.g. this is useful for putting in newlines (%0A) or carriage-returns (%0D), or otherwise storing arbitrary data. The two-character form, %FF, is of course only useful for encoding characters in the range 0 to 255. The multi-character form can be used for a hex number of any length, e.g. %{A}, %{23}, %{A9}, %{153}, %{201C}. See UNICODE HANDLING for more information.

This value is also used when serializing data. If true then the serialized data will have non-alphanumeric characters escaped.

IgnoreInvalidLines Default is 0. Disables warn()’ings that would normally occur when the parser encountered a line that couldn’t be understood or was invalid. Also disables the warning when ’AllowEmptyValues’ is false and you are attempting to serialize() an empty or undefined value.
IgnoreUnclosedTags Default is 0.

By default, should the configuration data have an unbalanced number of opening and closing tags, an error will be generated to this effect. If IgnoreUnclosedTags is set to 1 then this error will be downgraded to a warning.

Strict Default is 1.

By default any errors in the configuration will result in an error being thrown containing related details. To override this behaviour set the Strict option to 0, this will convert these errors into warnings and processing will continue.

Subs Default is 0. By default the configuration lines are read verbatim. However, sometimes you want to be able to pick data from the environment, or you want to set a common string e.g. at the top of the file or in the Variables option (see below). This re-use or interpolation of values can save lots of repetition, and improve portability of configuration files. This module implements this kind of interpolation and re-use by giving you the ability to pass each line through a templating engine.

Simply set this option to 1 to make every line pass through Any::Template (which is loaded on demand) before being parsed. As each line is read it is turned into a new Any::Template object, and then the process() method is given all of the configuration data that has been read so far, and whatever data was provided in the Variables option (see below).

Here’s an example of how you could use the feature, using a templating engine which looks in the data structure (mentioned above) and in the environment for its values. The template syntax is simply [INSERT variable] to insert a value, and let’s assume that the environment variable DOCROOT is set to ’/home/system’. So if Subs is true then the following lines:

        Colour = red
        @set FILE_SUFFIX cfg
        Filename        [INSERT DOCROOT]/data/testsite/[INSERT Colour]/main.[INSERT FILE_SUFFIX]

will be parsed into:

                Colour => red,
                Filename => /home/system/data/testsite/red/main.cfg

Obviously that’s a simple example but shows how this feature can be used to factor out common values. Your Any::Template-compatible templating engine may provide far more advanced features which you’re also free to use.

Note that keys in the Variables option override the keys derived from the configuration data so far. If the configuration data contains blocks then these will be available in the template’s data structure as the appropriate hash- or array-references, just as would be returned by the deserialize() method. Also note that after the templating step, the line may now actually contain line breaks - and if it does the parser will continue to work through each line, parsing each line separately. The current line will of course not be passed through the templating engine again, but any subsequent lines will be.

You can always use the Escapes feature to include unusual characters in your data if your templating engine is able to escape data in the right way.

After the templating step, the line is then parsed as usual. See the @reference directive (DIRECTIVES) for a related concept, where you can refer back to earlier values and blocks in their entirety.

TemplateBackend Only relevant if ’Subs’ is true. Choose which ’Backend’ to use with Any::Template. The default is empty, which means that Any::Template will use an environment variable to determine the default Backend - see Any::Template for details.
TemplateOptions Only relevant if ’Subs’ is true. Some Any::Template backends take a hash-reference as an ’Options’ constructor parameter. Set this option to the required hash-reference and it will be passed to the Any::Template constructor. Note that if the backend is changed by using a directive like ’@set TemplateBackend Foo’ this TemplateOptions will still be used.
UseQuotes Default is 1. If a value read from the config file is quoted (with matching or "), remove the quotes. Useful for including explicit whitespace. This option is also used when serializing data - if this option is true then values will always be written out with quotes.
Variables A reference to a hash which contains the names of some variables and their appropriate values. Only used when the Subs option is in effect. Note that this copied before use (using dclone() from Storable, loaded on demand) which means that the original data structure should be unaffected by @set directives, and that you can use the Config::Wrest object multiple times and the same data structure is used every time.
WriteWithEquals Default is 0. When serializing data, keys and values will be separated by ’=’.
WriteWithHeader Default is 1. When serializing data, the default behaviour is to emit lines at the start indicating the software that serialized the data and the specific settings of the AllowEmptyValues, Escapes, and UseQuotes directives. This option suppresses those lines.
WriteWithReferences Default is 0. If true then an appropriate ’@reference’ directive will be emitted during serialization whenever a perl data structure is referred to for the second, or subsequent, times.


The data is read line-by-line. Comments are stripped and blank lines are ignored. You can’t have multiple elements (key/value pairs, values in a list block, block opening tags, block closing tags, or directives) on a single line - you may only have one such element per line. Both the newline and carriage return characters (\n and \r) are considered as line breaks, and hence configuration files can be read and written across platforms (see UNICODE HANDLING).

Data is stored in two ways: as key/value pairs, or as individual values when inside a list block. Hash or list blocks may be nested inside other blocks to arbitrary depth.


Lines such as these are used at the top level of the configuration file, or inside HASH BLOCKS. The line simply has a key and a value, separated by whitespace or an ’=’ sign:

        name  =   "Scott Tiger"
        Age 23

The ’key’ can consist of \w characters, . and -. VALUE can include anything but a ’#’ to the end of the line. See Escapes and UseQuotes in CONSTRUCTOR OPTIONS.


Lines such as these are used inside LIST BLOCKS. The value is simply given:

        "Two Step"

These may not begin with these characters: ’[’, ’<’, ’(’, ’{’, ’:’, ’@’, ’%’, ’/’ because they are the first thing in a line and such characters would be confused with actual tags and reserved characters. See Escapes and UseQuotes in CONSTRUCTOR OPTIONS if your value begins with any of these, or if you want to include whitespace.


Comments may be on a line by themselves:

        # Next line is for marketing...
        Whiteness = Whizzy Whiteness!

or at the end of a line:

        Style=Loads of chrome     # thats what marketing want

Note that everything following a ’#’ character (in Unicode that’s called a NUMBER SIGN) is taken to be a comment, so if you want to have an actual ’#’ in your data you must have the Escapes option turned on (see CONSTRUCTOR OPTIONS) e.g.:

        Colour   %23FF9900

even if the ’#’ is in the middle of a quoted string:

        Foo "bar#baz" # a comment

is equivalent to:

        Foo "bar


A block which contains KEY VALUE PAIRS, or other blocks. They look like:

                colour red
                # contents go here

For convenience you can omit the block’s name in the closing tag, like this:

                Age 23
                # contents go here

The name of the block can consist of \w characters, . and -.


A block which contains a list of SINGLE VALUES, or other blocks. They look like:


and you can omit the name in the closing tag if you wish:

                # ...

The name of the block can consist of \w characters, . and -.


In KEY VALUE PAIRS the ’=’ between the Name and Value is optional, but it can have whitespace before and/or after it. If there’s no ’=’ you need whitespace to separate the Name and Value.

Block opening and closing tags cannot have whitespace inside them.

Lines may be indented by arbitrary whitespace. Trailing whitespace is stripped from values (but see the UseQuotes and Escapes entries in CONSTRUCTOR OPTIONS).


Sometimes you want to specify data with characters that are unprintable, hard-to type or have special meaning to Config::Wrest. You can escape such characters using two forms. Firstly, the ’%’ symbol followed by two hex digits, e.g. %A9, for characters up to 255 decimal. Secondly you can write ’%’ followed by any hex number in braces, e.g. %{201c} to specify any character by its Unicode code point. See ’Escapes’ under CONSTRUCTOR OPTIONS.


The configuration file itself can contain lines which tell the parser how to behave. All directive lines begin with an ’@’. For example you can turn on the URL-style escaping, you can set variables, and so on. These are recognized directives:
@include FILENAME Insert a file into the current configuration in place of this directive, and continue reading configuration information. This file is simply another file of Config::Wrest lines. If any options are set in the include, or in any nested includes, the effect of them will persist after the end of that file - i.e. when a file is included it is effectively merged with the parent file’s contents. The filename is treated according to the current setting of the UseQuotes and Escapes options.
@option NAME VALUE Allows you to alter the VALUE of the parser option called NAME that is otherwise set in the perl interface. See CONSTRUCTOR OPTIONS. The value is treated according to the current setting of the UseQuotes and Escapes options.
@reference [ NAME ] PATH Allows you to tell the parser to re-use a previous data value and put it in the current location against the given key ’NAME’ - inside hash blocks the ’NAME’ is required, but inside list blocks the ’NAME’ is optional and effectively ignored. This feature allows you to have a block or value in your config file which is re-used many times further on in the file. The ’NAME’ has the same restriction as for all other key names. The ’PATH’ is a string which specified the data item (which may be a plain value or a block) that you wish to reference, and is built up by joining a sequence of hash keys and array indexes together with ’->’ arrows. E.g. if you look at the example in DESCRIPTION then the path ’imageinfo->Nestedblock’ refers to that hash block, ’imageinfo->Nestedblock->colour’ refers the value ’red’, and ’Days->0’ is the value ’Sunday’. The ’PATH’ is treated according to the current setting of the UseQuotes and Escapes options.

Note that this is a different operation to using the ’Subs’ feature because this directive uses actual perl data references, rather than inserting some text which is then parsed into data structures, and hence can deal simply with complex structures. It is possible to construct circular data structures using this directive.

@set NAME VALUE Set a variable with the given NAME to any given VALUE, so that you may use that variable later on, if you’ve set the Subs option. The variable name must consist of alphanumeric and underscore characters only. The value is treated according to the current setting of the UseQuotes and Escapes options.


This section has been written from the point-of-view of perl 5.8, although the concepts translate to perl 5.6’s slightly different Unicode handling.

First it’s important to differentiate between configuration data that is given to deserialize() as a string which contains wide characters (i.e. code point >255), and data which contains escape sequences for wide characters. Escape sequences can only occur in certain places, whereas actual wide characters can be used in key names, block names, directives and in values. This is because the parser uses regular expressions which use metacharacters such as \w, and these can match against some wide characters.

Although you can use wide characters in directives, it may make no sense to try to @include a filename which contains wide characters.

Configuration data will generally be read to or written from a file at some stage. You should be aware that File::Slurp::WithinPolicy uses File::Slurp which reads files in byte-oriented fashion. If this is not what you want, e.g. if your config files contain multi-byte characters such as UTF8, then you should either read/write the file yourself using the appropriate layer in the arguments to open(), or use the Encode module to go between perl’s Unicode-based strings and the required encoding (e.g. your configuration files may be stored on disk as ISO-8859-1, but you want it to be read into perl as the Unicode characters, not as a stream of bytes). Similarly, you may wish to use Encode or similar to turn a string into the correct encoding for your application to use.

Unicode specifies a number of different characters that should be considered as line endings: not just u000A and u000D, but also u0085 and several others. However, to keep this module compatible with perl versions before 5.8 this module splits data into lines on the sequence \x0D\x0A <B>orB> on the regular expression /[\n\r]/, and does <B>notB> split on any of the other characters given in the Unicode standard. If you want your configuration data to use any of the other line endings you must read the file yourself, change the desired line ending to \n and pass that string to deserialize(). Reverse the process when using serialize() and writing files. E.g. on an OS/390 machine a configuration file may be stored with NEL (i.e. \x85) line endings which need to be changed when reading it on a Unix machine.

This module has not been tested on EBCDIC platforms.


If you try to deserialize configuration data that has the wrong syntax (e.g. mis-nested blocks, or too many closing tags) a fatal error will be raised.

Unrecognized directives cause a warning, as will key/value lines appearing in a list block, or list items appearing in a hash block (see AllowEmptyValues in CONSTRUCTOR OPTIONS). You also get a warning if there were too few closing tags and the parse implicitly closed some for you.


The data structure you want to serialize must be a hash reference. The values may be strings, arrayrefs or hashrefs, and so on recursively. Any bad reference types cause a fatal croak().

You are only allowed to use a restricted set of characters as hash keys, i.e. the names of block elements and the key in key/value pairs of data. If your data structure has a hash key that could create bad config data a fatal error is thrown with croak(). Values in list blocks are also checked, and a fatal error is raised if the value would create bad config data.

In general you will want to use the ’Escapes’ option described above. This makes it hard to produce bad configuration files.

If you want to dump out cyclic / self-referential data structures you’ll need to set the ’WriteWithReferences’ option, otherwise the deep recursion will be detected and the serialization will throw a fatal error.


parse_file(), write_file() and the ’@include’ directive load File::Slurp::WithinPolicy on demand to perform the file input/output operations. See perlunicode for more details on perl’s Unicode handling, and Encode for character recoding. See Any::Template, and the relevant templating modules, if the ’Subs’ option is true.

Although this module can read and write data structures it is not intended as an all-purpose serialization system. For that see Storable.

Unicode Newline Guidelines from


$Revision: 1.36 $ on $Date: 2006/08/22 14:09:50 $ by $Author: mattheww $


IF&L Software Engineers <cpan _at_ bbc _dot_ co _dot_ uk>


(c) BBC 2006. This program is free software; you can redistribute it and/or modify it under the GNU GPL.

See the file COPYING in this distribution, or

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