I haven't used a filter based on content much, due to getting frustrated trying to get it to work. I remember reading somewhere that wildcards are permissable, and my own experience suggests that they might be necessary in some circumstances. A link to relevant documentation eludes me for the moment.
edit: Google helped me locate a PDF file titled "Spark Business Mail User Guide." As near as I can make out, the webmail interface is similar to that used for Xtra accounts. There's a section "How can I use the conditions in E-Mail filter rules?" which mentions the meanings of the different filter options, and the use of wildcards.