NAME
Data::DPath - 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 hashesSee 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 quote 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 evals 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/BARbut can also happen inside paths to skip middle parts:
/AAA/BBB//FARAWAYThis allows any way between
BBBandFARAWAY. *-
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 stepFOOand the[2]reduces them to only the third element (index starts at 0). /FOO/*[ idx == 2 ]/-
The
*is a step that matches all elements afterFOO, 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
FOOit matches only those elements whose key is "CCC". /FOO/*[key =~ /CCC/ ]-
In all elements after step
FOOit 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
FOOthat have the valueRR2.Combine this with the parent step
..: //FOO/*[value eq "RR2"]/..-
Find such an element below
FOOwhere an element with valueRR2is contained. //FOO[size >= 3]-
Find
FOOelements 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). There's an equivalent attribute (idx) for the array index of a point stored in an array.
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)
SEE ALSO
There are other modules on CPAN which are related to finding elements in data structures.
AUTHOR
Steffen Schwigon <ss5@renormalist.net>
COPYRIGHT AND LICENSE
This software is copyright (c) 2024 by Steffen Schwigon.
This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.