Full regular expressions are not supported by testOnly. Wildcards are supported, however.
sbt> testOnly com.example.*Spec
Only the asterisk * is interpreted specially here and not the periods. This will select all tests beginning with com.example. and ending with Spec.
Or just all test Specs:
sbt> testOnly *Spec
testOnly and other testing information is documented here: http://www.scala-sbt.org/release/docs/Detailed-Topics/Testing