Find nodes that match a group of CSS selectors in an XML tree.
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.
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::", ...)
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.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.
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.
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.
Simon Potter
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"))