Looks dirty, but it works.
<button class="btn"
data-ng-class="{'btn-primary': modalOptions.actionButtonText.indexOf('delete') == -1,
'btn-danger': modalOptions.actionButtonText.indexOf('delete') > -1}"
data-ng-click="modalOptions.ok();">{{modalOptions.actionButtonText}}</button>
The ngClass directive lets you add/remove classes based on a boolean expression. So here, I am adding btn-primary
if "delete" isn't found in the button text and btn-danger
if it is.