protected function addAttribute(Tag $tag, $attrName)
	{
		$isUrl = false;
		$exprs = [];
		foreach ($this->captures as $key => $capture)
		{
			if ($capture['name'] !== $attrName)
			{
				continue;
			}
			$exprs[] = $capture['expr'];
			if (isset($this->references['asUrl'][$key]))
			{
				$isUrl = true;
			}
		}
		$exprs = array_unique($exprs);

		$regexp = $this->delimiter . '^';
		$regexp .= (count($exprs) === 1) ? $exprs[0] : '(?:' . implode('|', $exprs) . ')';
		$regexp .= '$' . $this->delimiter . 'D' . $this->modifiers;

		$attribute = $tag->attributes->add($attrName);

		$filter = $this->configurator->attributeFilters['#regexp'];
		$filter->setRegexp($regexp);
		$attribute->filterChain[] = $filter;

		if ($isUrl)
		{
			$filter = $this->configurator->attributeFilters['#url'];
			$attribute->filterChain[] = $filter;
		}
	}