system/Router/RouteCollection.php
public function setDefaultConstraint(string $placeholder): RouteCollectionInterface
	{
		if (array_key_exists($placeholder, $this->placeholders))
		{
			$this->defaultPlaceholder = $placeholder;
		}

		return $this;
	}
public function setDefaultConstraint(string $placeholder): RouteCollectionInterface
	{
		if (array_key_exists($placeholder, $this->placeholders))
		{
			$this->defaultPlaceholder = $placeholder;
		}

		return $this;
	}
Sets the default constraint to be used in the system. Typically
for use with the 'resource' method.

@param string $placeholder

@return RouteCollectionInterface