What's does m{} do ?

What does m{} do, like in the following test? body DRUG_DOSAGEm{[\d\.]+ *\$? *(?:[\\/]|per) *d.?o.?s.?e}i

Testing for short message?

Has anyone come up with a way to test for short messages? I.e., I want to test for short messages that only include a single URL. Thanks for any help. - Mark