Can you please advise on how to return a string between 2 specific characters?

### Example

> 
> `http://sds.01/create/http/capital/870745800/create/period_1_1364871116_438861511.ssa`
> 
> 
> 

I want to return : 1364871116\_438861511, which is between "period\_1\_" AND ".ssa"