Query XML with XPath

Goal: Select elements, attributes, and text from an XML file with a path expression such as //user[@role="dev"]/name/text(), instead of walking the tree by hand.

Prerequisites: The xpath.mq extension module. Copy xpath.mq into your module directory, place it anywhere and point at it with -L <dir>, or import it over HTTP with --allow-http-import and import "github.com/harehare/xpath.mq". Read the file with -I xml, so the query’s input is the parsed tree.

Query

The names of the users whose role is dev:

$ mq -I xml 'import "xpath" | xpath::xpath_query(., "//user[@role=\"dev\"]/name/text()")' users.xml

Input (users.xml)

<?xml version="1.0"?>
<users>
  <user id="1" role="admin"><name>Alice</name><email>[email protected]</email></user>
  <user id="2" role="dev"><name>Bob</name></user>
  <user id="3" role="dev"><name>Carol</name><email>[email protected]</email></user>
</users>

Output

["Bob", "Carol"]

More paths

Each of these runs against the same users.xml:

To get a single value instead of an array, use xpath_first, which returns None when there is no match:

$ mq -I xml 'import "xpath" | xpath::xpath_first(., "//user[@id=\"3\"]/name/text()")' users.xml

Carol

Notes

• xpath_query returns an array. An element step (//user) returns the element dicts, @name returns the attribute’s string, and text() returns the element’s text. Pass the elements to the xml module to keep working on them, for example map(fn(u): xml::xml_text(xml::xml_find(u, "name"));).
• No match returns [], which prints nothing in the default output format. Add -F json to see the [].
• This is an abbreviated XPath. It supports child, descendant (//), self, parent, * wildcards, @attr, text(), name(), positional and attribute or child predicates, and and, or, not(...), contains(...), starts-with(...). It does not implement the full XPath function library, so fall back to xml_find_all for anything else.
• A malformed path raises an error rather than returning [].
• For simple lookups by tag name, the built-in xml_find_all needs no extra module. See Read values from an XML file.