You can use the [CONTAINS][1]CONTAINS function as part a workflow rule or process builder.
Bsed on your requirement, you want a workflow rule that will update a custom field called House number when the value in the billing street field contains 'sh'
The formula is as follows: CONTAINS('sh',Partner_Custom_Field)
The formula return type should be a checkbox so it will return true or false and based on that the workflow will trigger.
Hope it helps. [1]: https://help.salesforce.com/apex/HTViewSolution?id=000004749&language=en_US