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Selects HTML elements using CSS 2 selectors.

The Selector class uses CSS selector expressions to match and select HTML elements.

For example:

selector = HTML::Selector.new "form.login[action=/login]"

creates a new selector that matches any form element with the class login and an attribute action with the value /login.

Matching Elements

Use the match method to determine if an element matches the selector.

For simple selectors, the method returns an array with that element, or nil if the element does not match. For complex selectors (see below) the method returns an array with all matched elements, of nil if no match found.

For example:

if selector.match(element)
  puts "Element is a login form"
end

Selecting Elements

Use the select method to select all matching elements starting with one element and going through all children in depth-first order.

This method returns an array of all matching elements, an empty array if no match is found

For example:

selector = HTML::Selector.new "input[type=text]"
matches = selector.select(element)
matches.each do |match|
  puts "Found text field with name #{match.attributes['name']}"
end

Expressions

Selectors can match elements using any of the following criteria:

  • name – Match an element based on its name (tag name). For example, p to match a paragraph. You can use * to match any element.

  • #id – Match an element based on its identifier (the id attribute). For example, #page.

  • .class – Match an element based on its class name, all class names if more than one specified.

  • [attr] – Match an element that has the specified attribute.

  • [attr=value] – Match an element that has the specified attribute and value. (More operators are supported see below)

  • :pseudo-class – Match an element based on a pseudo class, such as :nth-child and :empty.

  • :not(expr) – Match an element that does not match the negation expression.

When using a combination of the above, the element name comes first followed by identifier, class names, attributes, pseudo classes and negation in any order. Do not separate these parts with spaces! Space separation is used for descendant selectors.

For example:

selector = HTML::Selector.new "form.login[action=/login]"

The matched element must be of type form and have the class login. It may have other classes, but the class login is required to match. It must also have an attribute called action with the value /login.

This selector will match the following element:

<form class="login form" method="post" action="/login">

but will not match the element:

<form method="post" action="/logout">

Attribute Values

Several operators are supported for matching attributes:

  • name – The element must have an attribute with that name.

  • name=value – The element must have an attribute with that name and value.

  • name^=value – The attribute value must start with the specified value.

  • name$=value – The attribute value must end with the specified value.

  • name*=value – The attribute value must contain the specified value.

  • name~=word – The attribute value must contain the specified word (space separated).

  • name|=word – The attribute value must start with specified word.

For example, the following two selectors match the same element:

#my_id
[id=my_id]

and so do the following two selectors:

.my_class
[class~=my_class]

Alternatives, siblings, children

Complex selectors use a combination of expressions to match elements:

  • expr1 expr2 – Match any element against the second expression if it has some parent element that matches the first expression.

  • expr1 > expr2 – Match any element against the second expression if it is the child of an element that matches the first expression.

  • expr1 + expr2 – Match any element against the second expression if it immediately follows an element that matches the first expression.

  • expr1 ~ expr2 – Match any element against the second expression that comes after an element that matches the first expression.

  • expr1, expr2 – Match any element against the first expression, or against the second expression.

Since children and sibling selectors may match more than one element given the first element, the match method may return more than one match.

Pseudo classes

Pseudo classes were introduced in CSS 3. They are most often used to select elements in a given position:

  • :root – Match the element only if it is the root element (no parent element).

  • :empty – Match the element only if it has no child elements, and no text content.

  • :content(string) – Match the element only if it has string as its text content (ignoring leading and trailing whitespace).

  • :only-child – Match the element if it is the only child (element) of its parent element.

  • :only-of-type – Match the element if it is the only child (element) of its parent element and its type.

  • :first-child – Match the element if it is the first child (element) of its parent element.

  • :first-of-type – Match the element if it is the first child (element) of its parent element of its type.

  • :last-child – Match the element if it is the last child (element) of its parent element.

  • :last-of-type – Match the element if it is the last child (element) of its parent element of its type.

  • :nth-child(b) – Match the element if it is the b-th child (element) of its parent element. The value b specifies its index, starting with 1.

  • :nth-child(an+b) – Match the element if it is the b-th child (element) in each group of a child elements of its parent element.

  • :nth-child(-an+b) – Match the element if it is the first child (element) in each group of a child elements, up to the first b child elements of its parent element.

  • :nth-child(odd) – Match element in the odd position (i.e. first, third). Same as :nth-child(2n+1).

  • :nth-child(even) – Match element in the even position (i.e. second, fourth). Same as :nth-child(2n+2).

  • :nth-of-type(..) – As above, but only counts elements of its type.

  • :nth-last-child(..) – As above, but counts from the last child.

  • :nth-last-of-type(..) – As above, but counts from the last child and only elements of its type.

  • :not(selector) – Match the element only if the element does not match the simple selector.

As you can see, :nth-child pseudo class and its variant can get quite tricky and the CSS specification doesn't do a much better job explaining it. But after reading the examples and trying a few combinations, it's easy to figure out.

For example:

table tr:nth-child(odd)

Selects every second row in the table starting with the first one.

div p:nth-child(4)

Selects the fourth paragraph in the div, but not if the div contains other elements, since those are also counted.

div p:nth-of-type(4)

Selects the fourth paragraph in the div, counting only paragraphs, and ignoring all other elements.

div p:nth-of-type(-n+4)

Selects the first four paragraphs, ignoring all others.

And you can always select an element that matches one set of rules but not another using :not. For example:

p:not(.post)

Matches all paragraphs that do not have the class .post.

Substitution Values

You can use substitution with identifiers, class names and element values. A substitution takes the form of a question mark (?) and uses the next value in the argument list following the CSS expression.

The substitution value may be a string or a regular expression. All other values are converted to strings.

For example:

selector = HTML::Selector.new "#?", /^\d+$/

matches any element whose identifier consists of one or more digits.

See www.w3.org/TR/css3-selectors/

Methods
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Class Public methods
Selector.new(string, [values ...]) => selector

Creates a new selector from a CSS 2 selector expression.

The first argument is the selector expression. All other arguments are used for value substitution.

Throws InvalidSelectorError is the selector expression is invalid.

# File actionview/lib/action_view/vendor/html-scanner/html/selector.rb, line 241
def initialize(selector, *values)
  raise ArgumentError, "CSS expression cannot be empty" if selector.empty?
  @source = ""
  values = values[0] if values.size == 1 && values[0].is_a?(Array)

  # We need a copy to determine if we failed to parse, and also
  # preserve the original pass by-ref statement.
  statement = selector.strip.dup

  # Create a simple selector, along with negation.
  simple_selector(statement, values).each { |name, value| instance_variable_set("@#{name}", value) }

  @alternates = []
  @depends = nil

  # Alternative selector.
  if statement.sub!(/^\s*,\s*/, "")
    second = Selector.new(statement, values)
    @alternates << second
    # If there are alternate selectors, we group them in the top selector.
    if alternates = second.instance_variable_get(:@alternates)
      second.instance_variable_set(:@alternates, [])
      @alternates.concat alternates
    end
    @source << " , " << second.to_s
  # Sibling selector: create a dependency into second selector that will
  # match element immediately following this one.
  elsif statement.sub!(/^\s*\+\s*/, "")
    second = next_selector(statement, values)
    @depends = lambda do |element, first|
      if element = next_element(element)
        second.match(element, first)
      end
    end
    @source << " + " << second.to_s
  # Adjacent selector: create a dependency into second selector that will
  # match all elements following this one.
  elsif statement.sub!(/^\s*~\s*/, "")
    second = next_selector(statement, values)
    @depends = lambda do |element, first|
      matches = []
      while element = next_element(element)
        if subset = second.match(element, first)
          if first && !subset.empty?
            matches << subset.first
            break
          else
            matches.concat subset
          end
        end
      end
      matches.empty? ? nil : matches
    end
    @source << " ~ " << second.to_s
  # Child selector: create a dependency into second selector that will
  # match a child element of this one.
  elsif statement.sub!(/^\s*>\s*/, "")
    second = next_selector(statement, values)
    @depends = lambda do |element, first|
      matches = []
      element.children.each do |child|
        if child.tag? && subset = second.match(child, first)
          if first && !subset.empty?
            matches << subset.first
            break
          else
            matches.concat subset
          end
        end
      end
      matches.empty? ? nil : matches
    end
    @source << " > " << second.to_s
  # Descendant selector: create a dependency into second selector that
  # will match all descendant elements of this one. Note,
  elsif statement =~ /^\s+\S+/ && statement != selector
    second = next_selector(statement, values)
    @depends = lambda do |element, first|
      matches = []
      stack = element.children.reverse
      while node = stack.pop
        next unless node.tag?
        if subset = second.match(node, first)
          if first && !subset.empty?
            matches << subset.first
            break
          else
            matches.concat subset
          end
        elsif children = node.children
          stack.concat children.reverse
        end
      end
      matches.empty? ? nil : matches
    end
    @source << " " << second.to_s
  else
    # The last selector is where we check that we parsed
    # all the parts.
    unless statement.empty? || statement.strip.empty?
      raise ArgumentError, "Invalid selector: #{statement}"
    end
  end
end
Instance Public methods
Selector.for_class(cls) => selector

Creates a new selector for the given class name.

# File actionview/lib/action_view/vendor/html-scanner/html/selector.rb, line 216
def for_class(cls)
  self.new([".?", cls])
end
Selector.for_id(id) => selector

Creates a new selector for the given id.

# File actionview/lib/action_view/vendor/html-scanner/html/selector.rb, line 225
def for_id(id)
  self.new(["#?", id])
end
match(element, first?) => array or nil

Matches an element against the selector.

For a simple selector this method returns an array with the element if the element matches, nil otherwise.

For a complex selector (sibling and descendant) this method returns an array with all matching elements, nil if no match is found.

Use +first_only=true+ if you are only interested in the first element.

For example:

if selector.match(element)
  puts "Element is a login form"
end
# File actionview/lib/action_view/vendor/html-scanner/html/selector.rb, line 365
def match(element, first_only = false)
  # Match element if no element name or element name same as element name
  if matched = (!@tag_name || @tag_name == element.name)
    # No match if one of the attribute matches failed
    for attr in @attributes
      if element.attributes[attr[0]] !~ attr[1]
        matched = false
        break
      end
    end
  end

  # Pseudo class matches (nth-child, empty, etc).
  if matched
    for pseudo in @pseudo
      unless pseudo.call(element)
        matched = false
        break
      end
    end
  end

  # Negation. Same rules as above, but we fail if a match is made.
  if matched && @negation
    for negation in @negation
      if negation[:tag_name] == element.name
        matched = false
      else
        for attr in negation[:attributes]
          if element.attributes[attr[0]] =~ attr[1]
            matched = false
            break
          end
        end
      end
      if matched
        for pseudo in negation[:pseudo]
          if pseudo.call(element)
            matched = false
            break
          end
        end
      end
      break unless matched
    end
  end

  # If element matched but depends on another element (child,
  # sibling, etc), apply the dependent matches instead.
  if matched && @depends
    matches = @depends.call(element, first_only)
  else
    matches = matched ? [element] : nil
  end

  # If this selector is part of the group, try all the alternative
  # selectors (unless first_only).
  if !first_only || !matches
    @alternates.each do |alternate|
      break if matches && first_only
      if subset = alternate.match(element, first_only)
        if matches
          matches.concat subset
        else
          matches = subset
        end
      end
    end
  end

  matches
end
next_element(element, name = nil)

Returns the next element after this one. Skips sibling text nodes.

With the name argument, returns the next element with that name, skipping other sibling elements.

# File actionview/lib/action_view/vendor/html-scanner/html/selector.rb, line 495
def next_element(element, name = nil)
  if siblings = element.parent.children
    found = false
    siblings.each do |node|
      if node.equal?(element)
        found = true
      elsif found && node.tag?
        return node if (name.nil? || node.name == name)
      end
    end
  end
  nil
end
select(root) => array

Selects and returns an array with all matching elements, beginning with one node and traversing through all children depth-first. Returns an empty array if no match is found.

The root node may be any element in the document, or the document itself.

For example:

selector = HTML::Selector.new "input[type=text]"
matches = selector.select(element)
matches.each do |match|
  puts "Found text field with name #{match.attributes['name']}"
end
# File actionview/lib/action_view/vendor/html-scanner/html/selector.rb, line 455
def select(root)
  matches = []
  stack = [root]
  while node = stack.pop
    if node.tag? && subset = match(node, false)
      subset.each do |match|
        matches << match unless matches.any? { |item| item.equal?(match) }
      end
    elsif children = node.children
      stack.concat children.reverse
    end
  end
  matches
end
select_first(root)

Similar to select but returns the first matching element. Returns nil if no element matches the selector.

# File actionview/lib/action_view/vendor/html-scanner/html/selector.rb, line 473
def select_first(root)
  stack = [root]
  while node = stack.pop
    if node.tag? && subset = match(node, true)
      return subset.first if !subset.empty?
    elsif children = node.children
      stack.concat children.reverse
    end
  end
  nil
end
Instance Protected methods
attribute_match(equality, value)

Create a regular expression to match an attribute value based on the equality operator (=, ^=, |=, etc).

# File actionview/lib/action_view/vendor/html-scanner/html/selector.rb, line 689
def attribute_match(equality, value)
  regexp = value.is_a?(Regexp) ? value : Regexp.escape(value.to_s)
  case equality
    when "=" then
      # Match the attribute value in full
      Regexp.new("^#{regexp}$")
    when "~=" then
      # Match a space-separated word within the attribute value
      Regexp.new("(^|\s)#{regexp}($|\s)")
    when "^="
      # Match the beginning of the attribute value
      Regexp.new("^#{regexp}")
    when "$="
      # Match the end of the attribute value
      Regexp.new("#{regexp}$")
    when "*="
      # Match substring of the attribute value
      regexp.is_a?(Regexp) ? regexp : Regexp.new(regexp)
    when "|=" then
      # Match the first space-separated item of the attribute value
      Regexp.new("^#{regexp}($|\s)")
    else
      raise InvalidSelectorError, "Invalid operation/value" unless value.empty?
      # Match all attributes values (existence check)
      //
  end
end
next_selector(statement, values)

Called to create a dependent selector (sibling, descendant, etc). Passes the remainder of the statement that will be reduced to zero eventually, and array of substitution values.

This method is called from four places, so it helps to put it here for reuse. The only logic deals with the need to detect comma separators (alternate) and apply them to the selector group of the top selector.

# File actionview/lib/action_view/vendor/html-scanner/html/selector.rb, line 802
def next_selector(statement, values)
  second = Selector.new(statement, values)
  # If there are alternate selectors, we group them in the top selector.
  if alternates = second.instance_variable_get(:@alternates)
    second.instance_variable_set(:@alternates, [])
    @alternates.concat alternates
  end
  second
end
nth_child(a, b, of_type, reverse)

Returns a lambda that can match an element against the nth-child pseudo class, given the following arguments:

  • a – Value of a part.

  • b – Value of b part.

  • of_type – True to test only elements of this type (of-type).

  • reverse – True to count in reverse order (last-).

# File actionview/lib/action_view/vendor/html-scanner/html/selector.rb, line 724
def nth_child(a, b, of_type, reverse)
  # a = 0 means select at index b, if b = 0 nothing selected
  return lambda { |element| false } if a == 0 && b == 0
  # a < 0 and b < 0 will never match against an index
  return lambda { |element| false } if a < 0 && b < 0
  b = a + b + 1 if b < 0   # b < 0 just picks last element from each group
  b -= 1 unless b == 0  # b == 0 is same as b == 1, otherwise zero based
  lambda do |element|
    # Element must be inside parent element.
    return false unless element.parent && element.parent.tag?
    index = 0
    # Get siblings, reverse if counting from last.
    siblings = element.parent.children
    siblings = siblings.reverse if reverse
    # Match element name if of-type, otherwise ignore name.
    name = of_type ? element.name : nil
    found = false
    for child in siblings
      # Skip text nodes/comments.
      if child.tag? && (name == nil || child.name == name)
        if a == 0
          # Shortcut when a == 0 no need to go past count
          if index == b
            found = child.equal?(element)
            break
          end
        elsif a < 0
          # Only look for first b elements
          break if index > b
          if child.equal?(element)
            found = (index % a) == 0
            break
          end
        else
          # Otherwise, break if child found and count ==  an+b
          if child.equal?(element)
            found = (index % a) == b
            break
          end
        end
        index += 1
      end
    end
    found
  end
end
only_child(of_type)

Creates a only child lambda. Pass of-type to only look at elements of its type.

# File actionview/lib/action_view/vendor/html-scanner/html/selector.rb, line 774
def only_child(of_type)
  lambda do |element|
    # Element must be inside parent element.
    return false unless element.parent && element.parent.tag?
    name = of_type ? element.name : nil
    other = false
    for child in element.parent.children
      # Skip text nodes/comments.
      if child.tag? && (name == nil || child.name == name)
        unless child.equal?(element)
          other = true
          break
        end
      end
    end
    !other
  end
end
simple_selector(statement, values, can_negate = true)

Creates a simple selector given the statement and array of substitution values.

Returns a hash with the values tag_name, attributes, pseudo (classes) and negation.

Called the first time with can_negate true to allow negation. Called a second time with false since negation cannot be negated.

# File actionview/lib/action_view/vendor/html-scanner/html/selector.rb, line 522
def simple_selector(statement, values, can_negate = true)
  tag_name = nil
  attributes = []
  pseudo = []
  negation = []

  # Element name. (Note that in negation, this can come at
  # any order, but for simplicity we allow if only first).
  statement.sub!(/^(\*|[[:alpha:]][\w\-]*)/) do |match|
    match.strip!
    tag_name = match.downcase unless match == "*"
    @source << match
    "" # Remove
  end

  # Get identifier, class, attribute name, pseudo or negation.
  while true
    # Element identifier.
    next if statement.sub!(/^#(\?|[\w\-]+)/) do
      id = $1
      if id == "?"
        id = values.shift
      end
      @source << "##{id}"
      id = Regexp.new("^#{Regexp.escape(id.to_s)}$") unless id.is_a?(Regexp)
      attributes << ["id", id]
      "" # Remove
    end

    # Class name.
    next if statement.sub!(/^\.([\w\-]+)/) do
      class_name = $1
      @source << ".#{class_name}"
      class_name = Regexp.new("(^|\s)#{Regexp.escape(class_name)}($|\s)") unless class_name.is_a?(Regexp)
      attributes << ["class", class_name]
      "" # Remove
    end

    # Attribute value.
    next if statement.sub!(/^\[\s*([[:alpha:]][\w\-:]*)\s*((?:[~|^$*])?=)?\s*('[^']*'|"[^*]"|[^\]]*)\s*\]/) do
      name, equality, value = $1, $2, $3
      if value == "?"
        value = values.shift
      else
        # Handle single and double quotes.
        value.strip!
        if (value[0] == ?" || value[0] == ?') && value[0] == value[-1]
          value = value[1..-2]
        end
      end
      @source << "[#{name}#{equality}'#{value}']"
      attributes << [name.downcase.strip, attribute_match(equality, value)]
      "" # Remove
    end

    # Root element only.
    next if statement.sub!(/^:root/) do
      pseudo << lambda do |element|
        element.parent.nil? || !element.parent.tag?
      end
      @source << ":root"
      "" # Remove
    end

    # Nth-child including last and of-type.
    next if statement.sub!(/^:nth-(last-)?(child|of-type)\((odd|even|(\d+|\?)|(-?\d*|\?)?n([+\-]\d+|\?)?)\)/) do |match|
      reverse = $1 == "last-"
      of_type = $2 == "of-type"
      @source << ":nth-#{$1}#{$2}("
      case $3
        when "odd"
          pseudo << nth_child(2, 1, of_type, reverse)
          @source << "odd)"
        when "even"
          pseudo << nth_child(2, 2, of_type, reverse)
          @source << "even)"
        when /^(\d+|\?)$/  # b only
          b = ($1 == "?" ? values.shift : $1).to_i
          pseudo << nth_child(0, b, of_type, reverse)
          @source << "#{b})"
        when /^(-?\d*|\?)?n([+\-]\d+|\?)?$/
          a = ($1 == "?" ? values.shift :
               $1 == "" ? 1 : $1 == "-" ? -1 : $1).to_i
          b = ($2 == "?" ? values.shift : $2).to_i
          pseudo << nth_child(a, b, of_type, reverse)
          @source << (b >= 0 ? "#{a}n+#{b})" : "#{a}n#{b})")
        else
          raise ArgumentError, "Invalid nth-child #{match}"
      end
      "" # Remove
    end
    # First/last child (of type).
    next if statement.sub!(/^:(first|last)-(child|of-type)/) do
      reverse = $1 == "last"
      of_type = $2 == "of-type"
      pseudo << nth_child(0, 1, of_type, reverse)
      @source << ":#{$1}-#{$2}"
      "" # Remove
    end
    # Only child (of type).
    next if statement.sub!(/^:only-(child|of-type)/) do
      of_type = $1 == "of-type"
      pseudo << only_child(of_type)
      @source << ":only-#{$1}"
      "" # Remove
    end

    # Empty: no child elements or meaningful content (whitespaces
    # are ignored).
    next if statement.sub!(/^:empty/) do
      pseudo << lambda do |element|
        empty = true
        for child in element.children
          if child.tag? || !child.content.strip.empty?
            empty = false
            break
          end
        end
        empty
      end
      @source << ":empty"
      "" # Remove
    end
    # Content: match the text content of the element, stripping
    # leading and trailing spaces.
    next if statement.sub!(/^:content\(\s*(\?|'[^']*'|"[^"]*"|[^)]*)\s*\)/) do
      content = $1
      if content == "?"
        content = values.shift
      elsif (content[0] == ?" || content[0] == ?') && content[0] == content[-1]
        content = content[1..-2]
      end
      @source << ":content('#{content}')"
      content = Regexp.new("^#{Regexp.escape(content.to_s)}$") unless content.is_a?(Regexp)
      pseudo << lambda do |element|
        text = ""
        for child in element.children
          unless child.tag?
            text << child.content
          end
        end
        text.strip =~ content
      end
      "" # Remove
    end

    # Negation. Create another simple selector to handle it.
    if statement.sub!(/^:not\(\s*/, "")
      raise ArgumentError, "Double negatives are not missing feature" unless can_negate
      @source << ":not("
      negation << simple_selector(statement, values, false)
      raise ArgumentError, "Negation not closed" unless statement.sub!(/^\s*\)/, "")
      @source << ")"
      next
    end

    # No match: moving on.
    break
  end

  # Return hash. The keys are mapped to instance variables.
  {:tag_name=>tag_name, :attributes=>attributes, :pseudo=>pseudo, :negation=>negation}
end