QGIS » QGIS Application

1. Open attached layer
2. Properties → Labels → Rule-based labeling → Add rule (Filter: rstr('Special Speed Situation'), Label with: RSTR2)
3. Click on Test button: "Filter returned 3 feature(s)"

rstr - is my custom function added through Expression string builder:

import re

from qgis.core import *
from qgis.gui import *

@qgsfunction(args='auto', group='Custom')
def func(value1, feature, parent):
    return value1

def fvals(feature):
    return ((field.name(), feature.attribute(field.name())) for
        field in feature.fields())

@qgsfunction(args='auto', group='KartaBY')
def rstr(value1, feature, parent):
    pattern = r'^RSTR([1-9]|1[0-9]|2[0-9]|3[0-2])(_([1-9]|1[0-9]|2[0-9]|3[0-2]))?$'
    for name, value in fvals(feature):
        if (re.match(pattern, name) and
            value1.lower() in value.lower()):
            return True

As result - no labels on map.