SYNOPSIS

     use Data::CSel qw(csel);
    
     my @cells = csel("Table[name=~/data/i] TCell[value != '']:first", $tree);
    
     # ditto, but wrap result using a Data::CSel::Selection
     my $res = csel({wrap=>1}, "Table ...", $tree);
    
     # call method 'foo' of each node object (works even when there are zero nodes
     # in the selection object, or when some nodes do not support the 'foo' method
     $res->foo;

DESCRIPTION

    This module lets you use a query language (hereby named CSel) that is
    similar to CSS Selector to select nodes from a tree of objects.

EXPRESSION SYNTAX

    The following is description of the CSel query expression. It is
    modeled after the CSS Selector syntax with some modification (see
    "Differences with CSS selector").

    An expression is a chain of one or more selectors separated by commas.

    A selector is a chain of one or more simple selectors separated by
    combinators.

    A combinator is either: whitespace (descendant combinator), > (child
    combinator), ~ (general sibling combinator), or + (adjacent sibling
    combinator). E F, or two elements combined using descendant combinator,
    means F element descendant of an E element. E > F means F element child
    of E element. E ~ F means F element preceded by an E element. E + F
    means F element immediately preceded by an E element.

    A simple selector is either a type selector (see "Type selector") or
    universal selector (see "Universal selector") followed immediately by
    zero or more attribute selectors (see "Attribute selector" or class
    selector (see "Class selector"" in ") or ID selector (see "ID
    selector") or pseudo-classes (see "Pseudo-class"" in "), in any order.
    Type or universal selector is optional if there is at least one
    attribute selector or pseudo-class.

 Type selector

    A type selector is a Perl class/package name.

    Example:

     My::Class

    will match any My::Class object. Subclasses of My::Class will not be
    matched, use class selector for that.

 Universal selector

    A universal selector is * and matches any class/package.

    Example:

     *

    will match any object.

 Attribute selector

    An attribute selector filters objects based on the value of their
    attributes. The syntax is:

     [ATTR]
     [ATTR OP LITERAL]

    [ATTR] means to only select objects that have an attribute named ATTR,
    for example:

     [length]

    means to select objects that respond to (can()) length().

    Note: to select objects that do not have a specified attribute, you can
    use the :not pseudo-class (see "Pseudo-class"), for example:

     :not([length])

    [ATTR OP LITERAL] means to only select objects that have an attribute
    named ATTR that has value that matches the expression specified by
    operator OP and operand LITERAL.

    Experimental: a chain of attributes is allowed for the attribute, for
    example:

     [date.month = 12]

    will select only objects that has an attribute date, and the value of
    date is an object that has an attribute month, and the value of month
    is 12.

  Literal

    There are several kinds of literals supported.

    Numbers. Examples:

     1
     -2.3
     4.5e-6

    Boolean:

     true
     false

    Null (undef):

     null

    String. Either single-quoted (only recognizes the escape sequences \\
    and \'):

     'this is a string'
     'this isn\'t hard'

    or double-quoted (currently recognizes the escape sequences \\, \", \',
    \$ [literal $], \t [tab character], \n [newline], \r [linefeed], \f
    [formfeed], \b [backspace], \a [bell], \e [escape], \0 [null], octal
    escape e.g. \033, hexadecimal escape e.g. \x1b):

     "This is a string"
     "This isn't hard"
     "Line 1\nLine 2"

    For convenience, a word string can be unquoted in expression, e.g.:

     [name = ujang]

    is equivalent to:

     [name = 'ujang']

    Regex literal. Must be delimited by / ... /, can be followed by zero of
    more regex modifier characters m, s, i):

     //
     /ab(c|d)/i

  Operators

    The following are supported operators:

      * eq

      String equality using Perl's eq operator.

      Example:

       Table[title eq "TOC"]

      selects all Table objects that have title() with the value of "TOC".

      * = (or ==)

      Numerical equality using Perl's == operator.

      Example:

       TableCell[length=3]

      selects all TableCell objects that have length() with the value of 3.

      To avoid common trap, will switch to using Perl's eq operator when
      operand does not look like number, e.g.:

       Table[title = 'foo']

      is the same as:

       Table[title eq 'foo']

      * ne

      String inequality using Perl's ne operator.

      Example:

       Table[title ne "TOC"]

      selects all Table objects that have title() with the value not equal
      to "TOC".

      * != (or <>)

      Numerical inequality using Perl's != operator.

      Example:

       TableCell[length != 3]
       TableCell[length <> 3]

      selects all TableCell objects that have length() with the value not
      equal to 3.

      To avoid common trap, will switch to using Perl's ne operator when
      operand does not look like number, e.g.:

       Table[title != 'foo']

      is the same as:

       Table[title ne 'foo']

      * gt

      String greater-than using Perl's gt operator.

      Example:

       Person[first_name gt "Albert"]

      selects all Person objects that have first_name() with the value
      asciibetically greater than "Albert".

      * >

      Numerical greater-than using Perl's > operator.

      Example:

       TableCell[length > 3]

      selects all TableCell objects that have length() with the value
      greater than 3.

      To avoid common trap, will switch to using Perl's gt operator when
      operand does not look like number, e.g.:

       Person[first_name > 'Albert']

      is the same as:

       Person[first_name gt "Albert"]

      * ge

      String greater-than-or-equal-to using Perl's ge operator.

      Example:

       Person[first_name ge "Albert"]

      selects all Person objects that have first_name() with the value
      asciibetically greater than or equal to "Albert".

      * >=

      Numerical greater-than-or-equal-to using Perl's >= operator.

      Example:

       TableCell[length >= 3]

      selects all TableCell objects that have length() with the value
      greater than or equal to 3.

      To avoid common trap, will switch to using Perl's ge operator when
      operand does not look like number, e.g.:

       Person[first_name >= 'Albert']

      is the same as:

       Person[first_name ge "Albert"]

      * lt

      String less-than using Perl's lt operator.

      Example:

       Person[first_name lt "Albert"]

      selects all Person objects that have first_name() with the value
      asciibetically less than "Albert".

      * <

      Numerical less-than using Perl's < operator.

      Example:

       TableCell[length < 3]

      selects all TableCell objects that have length() with the value less
      than 3.

      To avoid common trap, will switch to using Perl's lt operator when
      operand does not look like number, e.g.:

       Person[first_name < 'Albert']

      is the same as:

       Person[first_name lt "Albert"]

      * le

      String less-than-or-equal-to using Perl's le operator.

      Example:

       Person[first_name le "Albert"]

      selects all Person objects that have first_name() with the value
      asciibetically less than or equal to "Albert".

      * <=

      Numerical less-than-or-equal-to using Perl's <= operator.

      Example:

       TableCell[length <= 3]

      selects all TableCell objects that have length() with the value less
      than or equal to 3.

      To avoid common trap, will switch to using Perl's le operator when
      operand does not look like number, e.g.:

       Person[first_name <= 'Albert']

      is the same as:

       Person[first_name le "Albert"]

      * =~ and !~

      Filter only objects where the attribute named attr has the value
      matching regular expression value. Operand should be a regex literal.
      Regex literal must be delimited by //.

      Example:

       Person[first_name =~ /^Al/]

      selects all Person objects that have first_name() with the value
      matching the regex /^Al/.

       Person[first_name =~ /^al/i]

      Same as previous example except the regex is case-insensitive.

      !~ is the opposite of =~, just like in Perl. It checks whether attr
      has value that does not match regular expression.

      * is and isnt

      Testing truth value or definedness. Value can be null or boolean
      literal.

      Example:

       DateTime[is_leap_year is true]

      will select all DateTime objects where its is_leap_year attribute has
      a true value.

       DateTime[is_leap_year is false]

      will select all DateTime objects where its is_leap_year attribute has
      a false value.

       Person[age isnt null]

      will select all Person objects where age is defined.

 Class selector

    A class selector is a . (dot) followed by Perl class/package name.

     .CLASSNAME

    It selects all objects that isa() a certain class. The difference with
    type selector is that inheritance is observed. So:

     .My::Class

    will match instances of My::Class as well as subclasses of it.

 ID selector

    An ID selector is a # (hash) followed by an identifier:

     #ID

    It is a special/shortcut form of attribute selector where the attribute
    is id and the operator is =:

     [id = ID]

    The csel() function allows you to configure which attribute to use as
    the ID attribute, the default is id.

 Pseudo-class

    A pseudo-class is : (colon) followed by pseudo-class name (a
    dash-separated word list), and optionally a list of arguments enclosed
    in parentheses.

     :PSEUDOCLASSNAME
     :PSEUDOCLASSNAME(ARG, ...)

    It filters result set based on some criteria. Currently supported
    pseudo-classes include:

      * :first

      Select only the first object from the result set.

      Example:

       Person[name =~ /^a/i]:first

      selects the first person whose name starts with the letter A.

      * :last

      Select only the last item from the result set.

      Example:

       Person[name =~ /^a/i]:last

      selects the last person whose name starts with the letter A.

      * :first-child

      Select only objects that are the first child of their parent.

      * :last-child

      Select only objects that are the last child of their parent.

      * :only-child

      Select only objects that is the only child of their parent.

      * :nth-child(n)

      Select only objects that are the nth child of their parent.

      * :nth-last-child(n)

      Select only objects that are the nth last child of their parent.

      * :first-of-type

      Select only objects that are the first child of their parent of their
      type. So if a parent's children is:

       id1(type=T1) id2(T2) id3(T2)

      then both id1 and id2 are first children of their respective types.

      * :last-of-type

      Select only objects that are the last child of their parent of their
      type.

      * :only-of-type

      Select only objects that are the only child of their parent of their
      type.

      * :nth-of-type(n)

      Select only objects that are the nth child of their parent of their
      type.

      * :nth-last-of-type(n)

      Select only objects that are the nth last child of their parent of
      their type.

      * :root

      Select only root node(s).

      * :empty

      Select only leaf node(s).

      * :not(S)

      Select all objects not matching selector S. S can be a string or an
      unquoted CSel expression.

      Example:

       :not('.My::Class')
       :not(.My::Class)

      will select all objects that are not of My::Class type.

      * :has(S)

      Select all objects that have a descendant matching selector S. S can
      be a string or an unquoted CSel expression.

      Example:

       :has('T')
       :not(T)

      will select all objects that have a descendant of type T.

 Differences with CSS selector

  Type selector can contain double colon (::)

    Since Perl package names are separated by ::, CSel allows it in type
    selector.

  Syntax of attribute selector is a bit different

    In CSel, the syntax of attribute selector is made simpler and more
    regular.

    There are operators not supported by CSel, but CSel adds more operators
    from Perl. In particular, the whole substring matching operations like
    [attr^=val], [attr$=val], [attr*=val], [attr~=val], and [attr|=val] are
    replaced with the more flexible regex matching instead [attr =~ /re/].

  Different pseudo-classes supported

    Some CSS pseudo-classes only make sense for a DOM or a visual browser,
    e.g. :link, :visited, :hover, so they are not supported.

  There is no concept of CSS namespaces

    CSS namespaces are used when there are foreign elements (e.g. SVG in
    addition to HTML) and one wants to use the same stylesheet for both.
    There is no need for something like this CSel, as we deal with only
    Perl objects.

FUNCTIONS

 csel([ \%opts , ] $expr, @tree_nodes) => list|selection_object

    Select from tree node objects @tree_nodes using CSel expression $expr.
    Will return a list of mattching node objects (unless when wrap option
    is true, in which case will return a Data::CSel::Selection object
    instead). Will die on errors (e.g. syntax error in expression, objects
    not having the required methods, etc).

    A tree node object is any regular Perl object satisfying the following
    criteria: 1) it supports a parent method which should return a single
    parent node object, or undef if object is the root node); 2) it
    supports a children method which should return a list (or an arrayref)
    of children node objects (where the list/array will be empty for a leaf
    node). Note: you can use Role::TinyCommons::Tree::Node to enforce this
    requirement.

    Known options:

      * class_prefixes => array of str

      Array of namespace to check when matching type in type selector as
      well as class selector. This is like PATH environment variable in
      Unix shell. For example, if class_prefixes is ["Foo::Bar", "Baz"],
      then this expression:

       T

      will match class Foo::Bar::T, or Baz::T, or T.

      * wrap => bool

      If set to true, instead of returning a list of matching nodes, the
      function will return a Data::CSel::Selection object instead (which
      wraps the result, for convenience). See the selection object's
      documentation for more details.

 parse_csel($expr) => hash|undef

    Parse an expression. On success, will return a hash containing parsed
    information. On failure, will return undef.

SEE ALSO

    CSS4 Selectors Specification, https://www.w3.org/TR/selectors4/.

    These modules let you use XPath (or XPath-like) syntax to select nodes
    of a data structure: Data::DPath. Like CSS selectors, XPath is another
    query language to select nodes of a document. XPath specification:
    https://www.w3.org/TR/xpath/.

    These modules let you use JSONPath syntax to select nodes of a data
    structure: JSON::Path. JSONPath is a query language to select nodes of
    a JSON document (data structure). JSONPath specification:
    http://goessner.net/articles/JsonPath.

    These modules let you use CSS selector syntax (or its subset) to select
    nodes of an HTML document: Mojo::DOM (or DOM::Tiny), jQuery, pQuery,
    HTML::Selector::XPath (or via Web::Query). The last two modules can
    also handle XPath expression.

