Add XPath syntax highlighter to Rouge

This is only used when building the site, like the Liquid extensions

docs/_plugins/xpath_lexer.rb

@@ -0,0 +1,66 @@
+# -*- coding: utf-8 -*- #
+
+require 'rouge'
+
+module Rouge
+  module Lexers
+    class XPath < RegexLexer
+
+      title "XPath"
+      desc "The XPath query language (http://en.wikipedia.org/wiki/XPath)"
+      tag 'xpath'
+      aliases 'xpath'
+      filenames '*.xpath'
+      mimetypes 'text/x-xpath'
+
+      state :basic do
+        rule /\s+/, Text
+        rule /[(][:](?![)])/, Comment, :nested_comment
+
+        rule /[\[\](){}|.,;!]/, Punctuation
+
+        rule /"[^"]*+"/, Str::Double
+        rule /'[^']*+'/, Str::Single
+
+        rule /\.\d++\b/, Num
+        rule /\b\d++\./, Num
+        rule /\b\d++(\.\d*+)?([eE][+-]?\d+)?\b/, Num
+      end
+
+      state :operators do
+        rule /(<|>|=<|>=|==|\/\/|[\/*+-])(?=\s|[a-zA-Z0-9\[])/, Operator
+        # operators
+        rule /(or|and|not|mod|ne|eq|lt|le|gt|ge)/, Operator
+        # keywords
+        rule /some|in|satisfies|as|is|for|every|cast|castable|treat|instance|of|to|if|then|else|return|let|intersect|except|union|div|idiv/, Keyword::Reserved
+        # axes
+        rule /(self|child|attribute|descendant|descendant-or-self|ancestor|ancestor-or-self|following|following-sibling|namespace|parent|preceding-sibling)::/, Keyword::Namespace
+        # kind tests
+        rule /(node|document-node|text|comment|namespace-node|processing-instruction|attribute|schema-attribute|element|schema-element|function)\(\)/, Keyword::Reserved
+        # functions
+        rule /[a-zA-Z\-]+(?=\()/, Name::Function
+      end
+
+      state :names do
+        rule /[a-zA-Z\-]+:/, Name::Namespace
+        rule /@[a-zA-Z][_\-a-zA-Z0-9]*/, Name::Attribute
+        rule /\$[a-zA-Z][_\-a-zA-Z0-9]*/, Name::Variable
+        # Node names
+        rule /[a-zA-Z]+/, Name::Tag
+      end
+
+      state :root do
+        mixin :basic
+        mixin :names
+        mixin :operators
+      end
+
+      state :nested_comment do
+        rule /[^(:)]+/, Comment
+        rule /\(:/, Comment, :push
+        rule /:\)/, Comment, :pop!
+        rule /[(:)]/, Comment
+      end
+    end
+  end
+end

pmd-doc/src/main/java/net/sourceforge/pmd/docs/RuleDocGenerator.java

@@ -391,7 +391,7 @@ public class RuleDocGenerator {
 
                     if (rule instanceof XPathRule || rule instanceof RuleReference && ((RuleReference) rule).getRule() instanceof XPathRule) {
                         lines.add("**This rule is defined by the following XPath expression:**");
-                        lines.add("```");
+                        lines.add("```xpath");
                         lines.addAll(toLines(StringUtils.stripToEmpty(rule.getProperty(XPathRule.XPATH_DESCRIPTOR))));
                         lines.add("```");
                     } else {