man Data::DPath (3): DPath is not XPath!

SYNOPSIS

use Data::DPath 'dpath';
my $data = {
AAA => { BBB => { CCC => [ qw/ XXX YYY ZZZ / ] },
RRR => { CCC => [ qw/ RR1 RR2 RR3 / ] },
DDD => { EEE => [ qw/ uuu vvv www / ] },
},
};

# Perl 5.8 style
my @resultlist = dpath('/AAA/*/CCC')->match($data); # ( ['XXX', 'YYY', 'ZZZ'], [ 'RR1', 'RR2', 'RR3' ] )

# Perl 5.10 style using overloaded smartmatch operator
my $resultlist = $data ~~ dpath '/AAA/*/CCC'; # [ ['XXX', 'YYY', 'ZZZ'], [ 'RR1', 'RR2', 'RR3' ] ]

Note that the "match()" function returns an array but the overloaded "~~" operator returns an array reference (that's a limitation of overloading).

Various other example paths from "t/data_dpath.t" (not neccessarily fitting to above data structure):

 $data ~~ dpath '/AAA/*/CCC'
 $data ~~ dpath '/AAA/BBB/CCC/../..'    # parents  (..)
 $data ~~ dpath '//AAA'                 # anywhere (//)
 $data ~~ dpath '//AAA/*'               # anywhere + anystep
 $data ~~ dpath '//AAA/*[size == 3]'    # filter by arrays/hash size
 $data ~~ dpath '//AAA/*[size != 3]'    # filter by arrays/hash size
 $data ~~ dpath '/"EE/E"/CCC'           # quote strange keys
 $data ~~ dpath '/AAA/BBB/CCC/*[1]'     # filter by array index
 $data ~~ dpath '/AAA/BBB/CCC/*[ idx == 1 ]' # same, filter by array index
 $data ~~ dpath '//AAA/BBB/*[key eq "CCC"]'  # filter by exact keys
 $data ~~ dpath '//AAA/*[ key =~ /CC/ ]'     # filter by regex matching keys
 $data ~~ dpath '//CCC/*[ value eq "RR2" ]'  # filter by values of hashes

See full details in "t/data_dpath.t".

You can get references into the $data data structure by using "dpathr":

 $data ~~ dpathr '//AAA/BBB/*'
 # etc.

You can request iterators to do incremental searches using "dpathi":

 my $benchmarks_iter = dpathi($data)->isearch("//Benchmark");
 while ($benchmarks_iter->isnt_exhausted)
 {
     my $benchmark = $benchmarks_iter->value;
     my $ancestors_iter = $benchmark->isearch ("/::ancestor");
     while ($ancestors_iter->isnt_exhausted)
     {
         my $ancestor = $ancestors_iter->value;
         print Dumper( $ancestor->deref );
     }
 }

This finds all elements anywhere behind a key ``Benchmark'' and for each one found print all its ancestors, respectively. See also chapter Iterator style.

ABOUT

With this module you can address points in a datastructure by describing a ``path'' to it using hash keys, array indexes or some wildcard-like steps. It is inspired by XPath but differs from it.

Why not XPath?

XPath is for XML. DPath is for data structures, with a stronger Perl focus.

Although XML documents are data structures, they are special.

Elements in XML always have an order which is in contrast to hash keys in Perl.

XML elements names on same level can be repeated, not so in hashes.

XML element names are more limited than arbitrary strange hash keys.

XML elements can have attributes and those can be addressed by XPath; Perl data structures do not need this. On the other side, data structures in Perl can contain blessed elements, DPath can address this.

XML has namespaces, data structures have not.

Arrays starting with index 1 as in XPath would be confusing to read for data structures.

DPath allows filter expressions that are in fact just Perl expressions not an own sub language as in XPath.

Comparison with Data::Path

There is a similar approach on CPAN, Data::Path. Here is a comparison matrix between Data::Path and Data::DPath.

(Warning: alpha grade comparison ahead, not yet fully verified, only evaluated by reading the source. Speed comparison not really benchmarked.)

 ---------------------------------------------------------------------
 Criteria             Data::Path           Data::DPath
 ---------------------------------------------------------------------
 
 real XPath syntax    no                   no
 
 ---------------------------------------------------------------------
 
 allow strange,       YES                  YES
 non-xml but
 perl-like            although
 hash keys            limited,
                      see next
 ---------------------------------------------------------------------
 
 allows special       no                   YES
 chars of own
 path syntax in                            you can quoting everything
 hash keys
 ("/[]|*.")
 
 ---------------------------------------------------------------------
 
 call subs in         YES                  no
 data structure,
 like:
 /method()
 ---------------------------------------------------------------------
 
 callbacks on         YES                  no
 not found keys
 
 ---------------------------------------------------------------------
 
 element "//"         no                   YES
 for "ANYWHERE"
 (//foo/bar)
 
 ---------------------------------------------------------------------
 
 element "."          no                   YES
 for "NOSTEP" or
 "actual position"
 (/.[filter expr])
 
 ---------------------------------------------------------------------
 
 element ".."         no                   YES
 for "PARENT"
 (//foo/..)
 
 ---------------------------------------------------------------------
 
 element "::ancestor" no                   YES
 for "ANCESTOR"
 (//foo/::ancestor)
 
 ---------------------------------------------------------------------
 
 element              no                   YES
 "::ancestor-or-self"
 
 ---------------------------------------------------------------------
 
 element "*"          no                   YES
 for "ANYSTEP" or
 "all subelements"
 (/foo/*)
 
 ---------------------------------------------------------------------
 
 array access         YES                  YES
 like /foo[4]
                      although             including negative indexes
                      limited              and whitespace awareness
 
 ---------------------------------------------------------------------
 
 complex              no                   YES
 filter expressions
 like                                      full Perl expressions
 /foo[size == 3] or                        plus sugar functions
 /.[isa("Foo::Bar")]
 
 ---------------------------------------------------------------------
 
 works with           YES                  YES
 blessed subelements
 
 ---------------------------------------------------------------------
 
 arrays start         YES                  YES
 with index 0
 (in contrast
 to 1 as in XPath)
 
 ---------------------------------------------------------------------
 
 array semantics      /foo[2]              /foo/*[2]
 is a bit different
 
 ---------------------------------------------------------------------
 
 handling of          croak                RETURN EMPTY
 not matching
 paths                but can be
                      overwritten
                      as callback
 
 ---------------------------------------------------------------------
 
 usage sugar          none                 overloaded '~~' operator
 
 ---------------------------------------------------------------------
 
 Speed                FAST                 quite fast
 
                      - raw Perl           - probably comparable
                      - considered fast      speed with expressions
                                             that Data::Path handles
                                           - slower on fuzzy paths,
                                             eg. with many "//" in it
 
 ---------------------------------------------------------------------
 
 Perl Versions        5.6+                 5.8+
 
 ---------------------------------------------------------------------
 
 Install chance       100%                 90%
 (http://deps
  .cpantesters
  .org)
 
 ---------------------------------------------------------------------

Summary

Generally Data::Path is for simpler use cases but does not suffer from surrounding meta problems: it has no dependencies, is fast and works on practically every Perl version.

Whereas Data::DPath provides more XPath-alike features, but isn't quite as fast and has more dependencies.

Security warning

Watch out! This module "eval"s parts of provided dpaths (in particular: the filter expressions). Don't use it if you don't trust your paths.

Since v0.41 the filter expressions are secured using Safe.pm to only allow basic Perl core ops. This provides more safety but is also significantly slower. To unrestrict this to pre-v0.41 raw "eval" behaviour you can set $Data::DPath::USE_SAFE to False:

  local $Data::DPath::USE_SAFE;
  # dpath '//CCC//*[ unsecure_perl_expression ]'

Read Safe.pm to understand how secure this is.

FUNCTIONS

dpath( $path_str )

Meant as the front end function for everyday use of Data::DPath. It takes a path string and returns a "Data::DPath::Path" object on which the match method can be called with data structures and the operator "~~" is overloaded.

The function is prototyped to take exactly one argument so that you can omit the parens in many cases.

See SYNOPSIS.

dpathr( $path_str )

Same as "dpath" but toggles that results are references to the matched points in the data structure.

dpathi( $data )

This is a different, iterator style, approach.

You provide the data structure on which to work and get back a current context containing the root element (as if you had searched for the path "/"), and now you can do incremental searches using "isearch".

See chapter Iterator style below for details.

API METHODS

match( $data, $path )

Returns an array of all values in $data that match the $path.

matchr( $data, $path )

Returns an array ref of all values in $data that match the $path.

OPERATOR

~~

Does a "match" of a dpath against a data structure.

Due to the matching nature of DPath the operator "~~" should make your code more readable.

THE DPATH LANGUAGE

Synopsis

 /AAA/BBB/CCC
 /AAA/*/CCC
 //CCC/*
 //CCC/*[2]
 //CCC/*[size == 3]
 //CCC/*[size != 3]
 /"EE/E"/CCC
 /AAA/BBB/CCC/*[1]
 /AAA/BBB/CCC/*[ idx == 1 ]
 //AAA/BBB/*[key eq "CCC"]
 //AAA/*[ key =~ /CC/ ]
 //CCC/*[value eq "RR2"]
 //.[ size == 4 ]
 /.[ isa("Funky::Stuff") ]/.[ size == 5 ]/.[ reftype eq "ARRAY" ]

Modeled on XPath

The basic idea is that of XPath: define a way through a datastructure and allow some funky ways to describe fuzzy ways. The syntax is roughly looking like XPath but in fact have not much more in common.

Some wording

I call the whole path a, well, path.

It consists of single (path) steps that are divided by the path separator "/".

Each step can have a filter appended in brackets "[]" that narrows down the matching set of results.

Additional functions provided inside the filters are called, well, filter functions.

Each step has a set of points relative to the set of points before this step, all starting at the root of the data structure.

Special elements

"//"

Anchors to any hash or array inside the data structure below the currently found points (or the root).

Typically used at the start of a path to anchor the path anywhere instead of only the root node:

  //FOO/BAR

but can also happen inside paths to skip middle parts:

 /AAA/BBB//FARAWAY

This allows any way between "BBB" and "FARAWAY".

"*"

Matches one step of any value relative to the current points (or the root). This step might be any hash key or all values of an array in the step before.

".."

Matches the parent element relative to the current points.

"::ancestor"

Matches all ancestors (parent, grandparent, etc.) of the current node.

"::ancestor-or-self"

Matches all ancestors (parent, grandparent, etc.) of the current node and the current node itself.

"."

A ``no step''. This keeps passively at the current points, but allows incrementally attaching filters to points or to otherwise hard to reach steps, like the top root element "/". So you can do:

 /.[ FILTER ]

or chain filters:

 /AAA/BBB/.[ filter1 ]/.[ filter2 ]/.[ filter3 ]

This way you do not need to stuff many filters together into one huge killer expression and can more easily maintain them.

See Filters for more details on filters.

If you need those special elements to be not special but as key names, just quote them:

 /"*"/
 /"*"[ filter ]/
 /"::ancestor"/
 /".."/
 /".."[ filter ]/
 /"."/
 /"."[ filter ]/
 /"//"/
 /"//"[ filter ]/

Difference between /step[filter] vs. /step/.[filter] vs. /step/*[filter]

The filter applies to the matched points of the step to which it is applied, therefore "/part[filter]" is the normal form, but see below how this affects array access.

The ``no step'' ``/.'' stays on the current step, therefore "/part/.[filter]" should be the same as "/part[filter]".

Lastly, "/part/*[filter]" means: take all the sub elements (``*'') below ``step'' and apply the filter to those. The most common use is to take ``all'' elements of an array and chose one element via index: "/step/*[4]/". This takes the fifth element of the array inside ``step''. This is explained in even more depth in the next section.

Difference between /affe[2] vs. /affe/*[2]

Read carefully. This is different from what you probably expect when you know XPath.

In XPath ``/affe[2]'' would address an item of all elements named ``affe'' on this step. This is because in XPath elements with the same name can be repeated, like this:

  <coolanimals>
    <affe>Pavian</affe>
    <affe>Gorilla</affe>
    <affe>Schimpanse</affe>
  </coolanimals>

and ``//affe[2]'' would get ``Schimpanse'' (we ignore the fact that in XPath array indexes start with 1, not 0 as in DPath, so we would actually get ``Gorilla''; anyway, both are funky fellows).

So what does ``/affe[2]'' return in DPath? Nothing! It makes no sense, because ``affe'' is interpreted as a hash key and hash keys can not repeat in Perl data structures.

So what you often want in DPath is to look at the elements below ``affe'' and takes the third of them, e.g. in such a structure:

 { affe => [
            'Pavian',
            'Gorilla',
            'Schimpanse'
           ]
 }

the path ``/affe/*[2]'' would return ``Schimpanse''.

Filters

Filters are conditions in brackets. They apply to all elements that are directly found by the path part to which the filter is appended.

Internally the filter condition is part of a "grep" construct (exception: single integers, they choose array elements). See below.

Examples:

"/FOO/*[2]/": A single integer as filter means choose an element from an array. So the "*" finds all subelements that follow current step "FOO" and the "[2]" reduces them to only the third element (index starts at 0).
"/FOO/*[ idx == 2 ]/": The "*" is a step that matches all elements after "FOO", but with the filter only those elements are chosen that are of index 2. This is actually the same as just "/FOO/*[2]".
"/FOO/*[key eq "CCC"]": In all elements after "FOO" it matches only those elements whose key is ``CCC''.
"/FOO/*[key =~ /CCC/ ]": In all elements after step "FOO" it matches only those elements whose key matches the regex "/CCC/". It is actually just Perl code inside the filter which works in a grep{}-like context.
"//FOO/*[value eq "RR2"]": Find elements below "FOO" that have the value "RR2".
Combine this with the parent step "..":
"//FOO/*[value eq "RR2"]/..": Find such an element below "FOO" where an element with value "RR2" is contained.
"//FOO[size >= 3]": Find "FOO" elements that are arrays or hashes of size 3 or bigger.

Filter functions

The filter condition is internally part of a "grep" over the current subset of values. So you can write any condition like in a grep and also use the variable $_.

Additional filter functions are available that are usually written to use $_ by default. See Data::DPath::Filters for complete list of available filter functions.

Here are some of them:

idx: Returns the current index inside array elements.
Please note that the current matching elements might not be in a defined order if resulting from anything else than arrays.
size: Returns the size of the current element. If it is an arrayref it returns number of elements, if it's a hashref it returns number of keys, if it's a scalar it returns 1, everything else returns -1.
key: Returns the key of the current element if it is a hashref. Else it returns undef.
value: Returns the value of the current element. If it is a hashref, return the value. If a scalar, return the scalar. Else return undef.

Special characters

There are 4 special characters: the slash "/", paired brackets "[]", the double-quote """ and the backslash "\". They are needed and explained in a logical order.

Path parts are divided by the slash "/".

A path part can be extended by a filter with appending an expression in brackets "[]".

To contain slashes in hash keys, they can be surrounded by double quotes """.

To contain double-quotes in hash keys they can be escaped with backslash "\".

Backslashes in path parts don't need to be escaped, except before escaped quotes (but see below on Backslash handling).

Filters of parts are already sufficiently divided by the brackets "[]". There is no need to handle special characters in them, not even double-quotes. The filter expression just needs to be balanced on the brackets.

So this is the order how to create paths:

1. backslash double-quotes that are part of the key
2. put double-quotes around the resulting key
3. append the filter expression after the key
4. separate several path parts with slashes

Backslash handling

If you know backslash in Perl strings, skip this paragraph, it should be the same.

It is somewhat difficult to create a backslash directly before a quoted double-quote.

Inside the DPath language the typical backslash rules of apply that you already know from Perl single quoted strings. The challenge is to specify such strings inside Perl programs where another layer of this backslashing applies.

Without quotes it's all easy. Both a single backslash "\" and a double backslash "\\" get evaluated to a single backslash "\".

Extreme edge case by example: To specify a plain hash key like this:

  "EE\E5\"

where the quotes are part of the key, you need to escape the quotes and the backslash:

  \"EE\E5\\\"

Now put quotes around that to use it as DPath hash key:

  "\"EE\E5\\\""

and if you specify this in a Perl program you need to additionally escape the backslashes (i.e., double their count):

  "\"EE\E5\\\\\\""

As you can see, strangely, this backslash escaping is only needed on backslashes that are not standing alone. The first backslash before the first escaped double-quote is ok to be a single backslash.

All strange, isn't it? At least it's (hopefully) consistent with something you know (Perl, Shell, etc.).

XPath idioms

Here are some typical XPath use-cases that can be achieved with Data::DPath, although a bit differently.

Attribute access

In XPath it's quite common to use a filter with attributes like this:

 //AAA/BBB/*[@CCC="DDD"]

A naive user could translate such a construct for Data::DPath like this:

 //AAA/BBB/*[CCC eq "DDD"]

except that it does not work. What works is this:

 //AAA/BBB/*[key eq "CCC" && value eq "DDD"]

Iterator style

The iterator style approach is an alternative to the already describe get-all-results-at-once approach. With it you iterate over the results one by one and even allow relative sub searches on each. The iterators use the Iterator API.

Please note, that the iterators do not save memory, they are just holding the context to go step-by-step and to start subsequent searches. Each iterator needs to evaluate its whole result set first. So in fact with nested iterators your memory might even go up.

Basic usage by example

Initialize a DPath iterator on a data structure using:

 my $root = dpathi($data);

Create a new iterator context, with the path relative to current root context:

 my $affe_iter = $root->isearch("//anywhere/affe");

Iterate over affe results:

 while ($affe_iter->isnt_exhausted)
 {
     my $affe_point = $affe_iter->value;     # next "affe" point
     my $affe       = $affe_point->deref;    # the actual "affe"
 }

Nested iterators example

This example is taken from the Benchmark::Perl::Formance suite, where the several plugins are allowed to provide their results anywhere at any level down in the result hash.

When the results are printed we look for all keys "Benchmark" and regenerate the path to each so we can name it accordingly, e.g., "plugin.name.subname".

For this we need an iterator to get the single "Benchmark" points one by one and evaluate the corresponding ancestors to fetch their hash keys. Here is the code:

 my $benchmarks_iter = dpathi($results)->isearch("//Benchmark");
 while ($benchmarks_iter->isnt_exhausted)
 {
     my $benchmark = $benchmarks_iter->value;
     my $ancestors_iter = $benchmark->isearch ("/::ancestor");
     while ($ancestors_iter->isnt_exhausted)
     {
         my $ancestor = $ancestors_iter->value;
         print Dumper( $ancestor->deref );               #(1)
         print $ancestor->first_point->{attrs}{key};     #(2)
     }
 }

Note that we have two iterators, the first one ($benchmarks_iter) over the actual benchmark results and the second one ($ancestors_iter) over the ancestors relative to one benchmark.

In line #(1) you can see that once you have the searched point, here the ancestors, you get the actual data using "$iterator->value->deref".

The line #(2) is utilizing the internal data structure to find out about the actual hash key under which the point is located. (There is also an official API to that: "$ancestor->first_point->attrs->key", but there it's necessary to check for undefined values before calling the methods attrs and key, so I went the easy way).

INTERNAL METHODS

To make pod coverage happy.

build_dpath

Prepares internal attributes for dpath.

build_dpathr

Prepares internal attributes for dpathr.

build_dpathi

Prepares internal attributes for dpathi.

AUTHOR

Steffen Schwigon, "<schwigon at cpan.org>"

CONTRIBUTIONS

Florian Ragwitz (cleaner exports, $_ scoping, general perl consultant)

AUTHOR

Steffen Schwigon <[email protected]>

COPYRIGHT AND LICENSE

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.

SYNOPSIS

ABOUT

Why not XPath?

Comparison with Data::Path

Security warning

FUNCTIONS

dpath( $path_str )

dpathr( $path_str )

dpathi( $data )

API METHODS

match( $data, $path )

matchr( $data, $path )

OPERATOR

~~

THE DPATH LANGUAGE

Synopsis

Modeled on XPath

Special elements

Difference between /step[filter] vs. /step/.[filter] vs. /step/*[filter]

Difference between /affe[2] vs. /affe/*[2]

Filters

Filter functions

Special characters

Backslash handling

XPath idioms

Iterator style

Basic usage by example

Nested iterators example

INTERNAL METHODS

build_dpath

build_dpathr

build_dpathi

AUTHOR

CONTRIBUTIONS

AUTHOR

COPYRIGHT AND LICENSE

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