Steps to reproduce:

Have Web Scenario, which generates several items. Those cannot be edited directly, but can be referenced in Triggers and newly created items - just fine.

One of them is: web.test.error[Get-Status], which contains text string of last error.
Error text string is similar to: "Could not resolve host: Could not resolve host: '127.0.0.1/TestValue=84'."

Then I create item:
type: Calculated Item
key: LevelValue
value type: Numeric or float

Formula:
tried both, this:

last("web.test.error[Get-Status]")

and even this:

last(\"web.test.error[Get-Status]\")

Pre-Processing:

I have tried both:

Reg-exp:  ^.+([0-9]+$
Value: \1

OR

Trim Left: 69
Trim Right: 2

Result:

Item not supported.
Error:
Cannot evaluate expression: expected numeric token at "Could not resolve host: Could not resolve host: '127.0.0.1/TestValue=84'.".

Expected:

Expected that this calculated item would strip-out numeric value from another item, a have it stored as numeric item.

Could anyone explain me, WHY there are pre-processing options like "regexp" and "trim" in "calculated item" type, if those simply do not work?
As the value of "last("web.test.error[...]") is evaluated against Numeric data type - BEFORE Pre-Processing is performed?
What the point of pre-processing then?
If it doesn't work before that numeric data type evaluation.