The spec specifies that the pattern parameter attributes should be a ECMA 262/Perl 5 regular expression. Alas, the spec does not specify whether expression enclosed in // are acceptable. None of the examples make use of // so I assume they are not allowed.
That then leads into whether the spec provides any support for expression modifiers, such as for case insensitivity. These would normally be written as flags after the closing /.
Does RAML support regexp modifiers?