Steps to reproduce:

1. Open "Create item prototype" form.
2. Enter some name and key.
3. Navigate to preprocessing tab.
4. Add preprocessing step with:
   • Type: Prometheus pattern.
   • 1st parameter: {{#M}.regsub("^([0-9]+)", "{#M}: \1")}
5. Press add button.

Result:
Unable to add the item prototype with macro function.

Expected:
No error occurs. Item prototype was successfully added.