Hi,
1. It is wrong syntax; there are two slashes missing
regexMatch("/^\+31\d{9}$/",Q1_other)

2. The main issue.
The leading "+" is not stored. There are some other threads about this here.
 
As the "+31" is a constant value you may only show it in front of the entry field with
regexMatch("/^\d{9}$/",Q1_other)
 
 

Or you force a "0031"
 

Or you redesign the question and use an inputmask.

Joffm