In using Xpath, how can I get only a part of the string value of the node?

This is my code..

<details>
  <detail status="ok">
  <elapsed>0</elapsed>
  <msg>Order successfully created. Order number is [100000017389]</msg>
  </detail>
</details>

This is part of the code I get from the test output.

I want to get that "[10000017389]" value, which varies in each time a test output is produced, while the "Order successfully created. Order number is " value is always constant.

So if I illustrate this carefully.. I cut the string value into two parts.

Part 1 = "Order successfully created. Order number is "

Part 2 = "[10000017389]"

Now my question is, how would my Xpath syntax look like when I want to use Part 1 to get Part 2 only?