The “Commons link” constraint check validates, among other things, that the value is “well-formed”:

• with no underscores instead of spaces
• with no unprintable characters at the beginning of the link
• with no leading or trailing spaces
• with no "%20" instead of spaces
• with no "Jump to: navigation, search" that sometimes gets pasted along
• that does not include the namespace (e. g. "File:", "Category:" and etc.)
• that does not include the full path from Commons (sample: "http://upload.wikimedia.org/wikipedia/commons/1/16/")

Some of these are caught by the existing validators for commonsMedia, but checks like “no underscores instead of spaces” are not included. The parser for commonsMedia should normalize the title (e. g. parse and format it: test_Image.jpg → Test Image.jpg), and the validator should check that the title is normalized.

Values for geo-shape and tabular-data should also be normalized, but in their case the value includes the namespace, while for commonsMedia the namespace is not part of the value.