selectorToXPath.js

// from http://www.joehewitt.com/blog/files/getElementsBySelector.js
// modified
function convertSelectorToXPath (rule, start) {
	var regElement    = /^([#.]?)([a-z0-9\\*_-]*)((\|)([a-z0-9\\*_-]*))?/i;
	var regAttr1      = /^\[([^\]]*)\]/i;
	var regAttr2      = /^\[\s*([^~=\s]+)\s*(~?=)\s*"([^"]+)"\s*\]/i;
	var regPseudo     = /^:([a-z_-])+/i;
	var regCombinator = /^(\s*[>+\s])?/i;
	var regComma      = /^\s*,/i;

	var index = 1;
	var parts = [".//", "*"];
	var lastRule = null;

	while (rule.length && rule != lastRule) {
		lastRule = rule;

		// Trim leading whitespace
		rule = rule.replace(/^\s*|\s*$/g,"");
		if (!rule.length) break;

		// Match the element identifier
		var m = regElement.exec(rule);
		if (m) {
			if (!m[1]) {
				// XXXjoe Namespace ignored for now
				if (m[5])
					parts[index] = m[5];
				else
					parts[index] = m[2];
			}
			else if (m[1] == '#')
				parts.push("[@id='" + m[2] + "']");
			else if (m[1] == '.')
				parts.push("[contains(concat(' ', @class, ' '), concat(' ', '" + m[2] + "', ' '))]");

			rule = rule.substr(m[0].length);
		}

		// Match attribute selectors
		m = regAttr2.exec(rule);
		if (m) {
			if (m[2] == "~=")
				parts.push("[contains(concat(' ', " + m[1] + ", ' '), concat(' ', '" + m[2] + "', ' '))]");
			else
				parts.push("[@" + m[1] + "='" + m[3] + "']");

			rule = rule.substr(m[0].length);
		} else {
			m = regAttr1.exec(rule);
			if (m) {
				parts.push("[@" + m[1] + "]");
				rule = rule.substr(m[0].length);
			}
		}

		// Skip over pseudo-classes and pseudo-elements, which are of no use to us
		m = regPseudo.exec(rule);
		while (m) {
			rule = rule.substr(m[0].length);
			m = regPseudo.exec(rule);
		}

		// Match combinators
		m = regCombinator.exec(rule);
		if (m && m[0].length) {
			if (m[0].indexOf(">") != -1)
				parts.push("/");
			else if (m[0].indexOf("+") != -1)
				parts.push("/following-sibling::");
			else
				parts.push("//");

			index = parts.length;
			parts.push("*");
			rule = rule.substr(m[0].length);
		}

		m = regComma.exec(rule);
		if (m) {
			parts.push(" | ", ".//", "*");
			index = parts.length-1;
			rule = rule.substr(m[0].length);
		}
	}

	var xpath = parts.join("");
	return xpath;
}

function $S (selector, context, type) {
	var xpath = convertSelectorToXPath(selector);
	return $X(xpath, context, type);
}