I looked through the tests and previous versions to see if that pipe was a character to match or a misunderstood "or" operator.  It was introduced in this [commit](https://github.com/angular/angular.js/commit/04cebcc133c8b433a3ac5f72ed19f3631778142b#diff-34e732507a1b6941aaf4ae5084a7c6ca) and from the lines assigning only `star` and `optional` along with no use of the matched pipe character within that block it appears to be the latter.