XPath is a Ruby DSL around a subset of XPath 1.0. Its primary purpose is to facilitate writing complex XPath queries from Ruby code.

To create quick, one-off expressions, XPath.generate can be used:

XPath.generate { |x| x.descendant(:ul)[x.attr(:id) == 'foo'] }

You can also call expression methods directly on the XPath module:

XPath.descendant(:ul)[XPath.attr(:id) == 'foo']

However for more complex expressions, it is probably more convenient to include the XPath module into your own class or module:

module MyXPaths
  include XPath

  def foo_ul
    descendant(:ul)[attr(:id) == 'foo']
  end

  def password_field(id)
    descendant(:input)[attr(:type) == 'password'][attr(:id) == id]
  end
end

Both ways return an XPath::Expression instance, which can be further modified. To convert the expression to a string, just call #to_s on it. All available expressions are defined in XPath::DSL.

When you send a string as an argument to any XPath function, XPath assumes this to be a string literal. On the other hand if you send in Symbol, XPath assumes this to be an XPath literal. Thus the following two statements are not equivalent:

XPath.descendant(:p)[XPath.attr(:id) == 'foo']
XPath.descendant(:p)[XPath.attr(:id) == :foo]

These are the XPath expressions that these would be translated to:

.//p[@id = 'foo']
.//p[@id = foo]

The second expression would match any p tag whose id attribute matches a 'foo' tag it contains. Most likely this is not what you want.

In fact anything other than a String is treated as a literal. Thus the following works as expected:

XPath.descendant(:p)[1]

Keep in mind that XPath is 1-indexed and not 0-indexed like most other programming languages, including Ruby.

Hashes are automatically converted to equality expressions, so the above example could be written as:

XPath.descendant(:p)[:@id => 'foo']

Which would generate the same expression:

.//p[@id = 'foo']

Note that the same rules apply here, both the keys and values in the hash are treated the same way as any other expression in XPath. Thus the following are not equivalent:

XPath.descendant(:p)[:@id => 'foo'] # => .//p[@id = 'foo']
XPath.descendant(:p)[:id => 'foo']  # => .//p[id = 'foo']
XPath.descendant(:p)['id' => 'foo'] # => .//p['id' = 'foo']

XPath comes with a set of premade XPaths for use with HTML documents.

You can generate these like this:

XPath::HTML.link('Home')
XPath::HTML.field('Name')

See XPath::HTML for all available matchers.

(The MIT License)

Copyright © 2010 Jonas Nicklas

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