Package 'selectr'

Translate a CSS selector to an equivalent XPath expression.

Description

This function aims to create an XPath expression equivalent to what would be matched by the given CSS selector. The reason the translation is required is because the XML and xml2 packages, being a libxml2 wrappers, can only evaluate XPath expressions.

Using this function, it is possible to search an XML tree without the prerequisite of knowing XPath.

Usage

css_to_xpath(selector,
             prefix = "descendant-or-self::",
             translator = "generic")

Arguments

selector	A character vector of CSS selectors.
prefix	The prefixes to apply to the resulting XPath expressions. The default or "" are most commonly used.
translator	The type of translator that will be used. Possible options are generic (the default), or html or xhtml.

Details

Each selector given to this function will be translated to an equivalent XPath expression. The resulting XPath expression can be given a prefix which determines the scope of the expression. The default prefix determines the scope to be the node itself and all descendants of the node. Most commonly the prefix is either the default or "", unless it is known what scope a particular XPath expression should have.

The translator used is usually unnecessary to specify as the default is sufficient for most cases. However, it is of use when creating expressions relating to (X)HTML pseudo elements and languages. In particular it qualifies the following pseudo selectors to apply only to relevant (X)HTML elements: :checked, :disabled, :enabled and :link.

When the translator is set to html, all elements and attributes will be converted to lower case. This restriction is removed when the translator is xhtml (or the default generic translator).

Value

A character vector of XPath expressions.

Author(s)

Simon Potter

References

CSS3 Selectors https://www.w3.org/TR/css3-selectors/, XPath https://www.w3.org/TR/xpath/.

Examples

css_to_xpath(".testclass")
  css_to_xpath("#testid", prefix = "")
  css_to_xpath("#testid .testclass")
  css_to_xpath(":checked", translator = "html")

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Find nodes that match a group of CSS selectors in an XML tree.

Description

The purpose of these functions is to mimic the functionality of the querySelector and querySelectorAll functions present in Internet browsers. This is so we can succinctly query an XML tree for nodes matching a CSS selector.

Namespaced functions querySelectorNS and querySelectorAllNS are also provided to search relative to a given namespace.

Usage

querySelector(doc, selector, ns = NULL, ...)
querySelectorAll(doc, selector, ns = NULL, ...)
querySelectorNS(doc, selector, ns,
                prefix = "descendant-or-self::", ...)
querySelectorAllNS(doc, selector, ns,
                   prefix = "descendant-or-self::", ...)

Arguments

doc	The XML document or node to be evaluated against.
selector	A selector used to query doc. This must be a single character string.
ns	The namespace that the query will be filtered to. This is a named list or vector which has as its name a namespace, and its value is the namespace URI. This can be ignored for the un-namespaced functions.
prefix	The prefix to apply to the resulting XPath expression. The default or "" are most commonly used.
...	Parameters to be passed onto css_to_xpath.

Details

The querySelectorNS and querySelectorAllNS functions are convenience functions for working with namespaced documents. They filter out all content that does not belong within the given namespaces. Note that when searching for particular elements in a selector, they must have a namespace prefix, e.g. "svg|g".

The namespace argument, ns, is simply passed on to getNodeSet or xml_find_all if it is necessary to use a namespace present within the document. This can be ignored for content lacking a namespace, which is usually the case when using querySelector or querySelectorAll.

Value

For querySelector, the result is a single node that represents the first matched node from a selector. If no matching nodes are found, NULL is returned.

For querySelectorAll, the result is a list of XML nodes. This list may be empty in the case that no match is found.

The querySelectorNS and querySelectorAllNS functions return the same type of content as their un-namespaced counterparts.

Author(s)

Simon Potter

References

CSS3 Selectors https://www.w3.org/TR/css3-selectors/, XPath https://www.w3.org/TR/xpath/, querySelectorAll https://developer.mozilla.org/en-US/docs/DOM/Document.querySelectorAll and http://www.w3.org/TR/selectors-api/#interface-definitions.

Examples

hasXML <- require(XML)
  hasxml2 <- require(xml2)

  if (!hasXML && !hasxml2)
    return() # can't demo without XML or xml2 packages present

  parseFn <- if (hasXML) xmlParse else read_xml
  # Demo for working with the XML package (if present, otherwise xml2)
  exdoc <- parseFn('<a><b class="aclass"/><c id="anid"/></a>')
  querySelector(exdoc, "#anid")   # Returns the matching node
  querySelector(exdoc, ".aclass") # Returns the matching node
  querySelector(exdoc, "b, c")    # First match from grouped selection
  querySelectorAll(exdoc, "b, c") # Grouped selection
  querySelectorAll(exdoc, "b")    # A list of length one
  querySelector(exdoc, "d")       # No match
  querySelectorAll(exdoc, "d")    # No match

  # Read in a document where two namespaces are being set:
  # SVG and MathML
  svgdoc <- parseFn(system.file("demos/svg-mathml.svg",
                                package = "selectr"))
  # Search for <script/> elements in the SVG namespace
  querySelectorNS(svgdoc, "svg|script",
                  c(svg = "http://www.w3.org/2000/svg"))
  querySelectorAllNS(svgdoc, "svg|script",
                     c(svg = "http://www.w3.org/2000/svg"))
  # MathML content is *within* SVG content,
  # search for <mtext> elements within the MathML namespace
  querySelectorNS(svgdoc, "math|mtext",
                  c(math = "http://www.w3.org/1998/Math/MathML"))
  querySelectorAllNS(svgdoc, "math|mtext",
                     c(math = "http://www.w3.org/1998/Math/MathML"))
  # Search for *both* SVG and MathML content
  querySelectorAllNS(svgdoc, "svg|script, math|mo",
                     c(svg = "http://www.w3.org/2000/svg",
                       math = "http://www.w3.org/1998/Math/MathML"))

  if (!hasXML)
    return() # already demo'd xml2

  # Demo for working with the xml2 package
  exdoc <- read_xml('<a><b class="aclass"/><c id="anid"/></a>')
  querySelector(exdoc, "#anid")   # Returns the matching node
  querySelector(exdoc, ".aclass") # Returns the matching node
  querySelector(exdoc, "b, c")    # First match from grouped selection
  querySelectorAll(exdoc, "b, c") # Grouped selection
  querySelectorAll(exdoc, "b")    # A list of length one
  querySelector(exdoc, "d")       # No match
  querySelectorAll(exdoc, "d")    # No match

  # Read in a document where two namespaces are being set:
  # SVG and MathML
  svgdoc <- read_xml(system.file("demos/svg-mathml.svg",
                                 package = "selectr"))
  # Search for <script/> elements in the SVG namespace
  querySelectorNS(svgdoc, "svg|script",
                  c(svg = "http://www.w3.org/2000/svg"))
  querySelectorAllNS(svgdoc, "svg|script",
                     c(svg = "http://www.w3.org/2000/svg"))
  # MathML content is *within* SVG content,
  # search for <mtext> elements within the MathML namespace
  querySelectorNS(svgdoc, "math|mtext",
                  c(math = "http://www.w3.org/1998/Math/MathML"))
  querySelectorAllNS(svgdoc, "math|mtext",
                     c(math = "http://www.w3.org/1998/Math/MathML"))
  # Search for *both* SVG and MathML content
  querySelectorAllNS(svgdoc, "svg|script, math|mo",
                     c(svg = "http://www.w3.org/2000/svg",
                       math = "http://www.w3.org/1998/Math/MathML"))

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