Steps to reproduce:
1. Have HOST with 2 items: Numeric and Text
2. Create trigger
2.1 Open expression popup
2.1.1 Choose Text item
Note that there are many string functions, for example replace():

2.1.2 Choose Numeric item
Note that most of string functions are not available, including replace() function:

3. Close expression popup without adding any function
4. Enter expression manually, but using string function for Numeric item:

replace(last(/HOST/numeric),"pattern","replace")=0

Result:
Trigger is successfully saved.
Expected:
Error: incorrect expression for numeric item.