/** The key of the configuration. e.g. //LINK[@rel='canonical'][1]/@href */
    String CRAWLER_DOCUMENT_HTML_CANONICAL_XPATH = "crawler.document.html.canonical.xpath";

    /** The key of the configuration. e.g. noscript,script,style,header,footer,aside,nav,a[rel=nofollow] */
    String CRAWLER_DOCUMENT_HTML_PRUNED_TAGS = "crawler.document.html.pruned.tags";

</p>

<pre class="ebnf">
Expression = UnaryExpr | Expression binary_op Expression .
UnaryExpr  = PrimaryExpr | unary_op UnaryExpr .

binary_op  = "||" | "&amp;&amp;" | rel_op | add_op | mul_op .
rel_op     = "==" | "!=" | "&lt;" | "&lt;=" | ">" | ">=" .
add_op     = "+" | "-" | "|" | "^" .
mul_op     = "*" | "/" | "%" | "&lt;&lt;" | "&gt;&gt;" | "&amp;" | "&amp;^" .