Tamper plugin conditions return value instead of boolean

Well, those conditions are an extension of the "Compare 2 arbitrary values" condition. That means, the eca_tamper conditions compare the original and the tampered value with each other. That happens in \Drupal\eca_tamper\Plugin\ECA\Condition\Tamper::getLeftValue and \Drupal\eca_tamper\Plugin\ECA\Condition\Tamper::getRightValue. The right value is the original one and the left value is the tampered value. The comparison then happens in \Drupal\eca\Plugin\ECA\Condition\StringComparisonBase::evaluate and that return TRUE or FALSE already.

So, this works as designed.

However, there is one special case when the tampering responds with an exception while determining the left value. In that case, we currently set the left value to the same as the original one, assuming that if the tampering isn't working, then the value doesn't get tampered.

Question: are you only seeing this problem if the regex runs into an error?

Thanks for the explanation, that helped me to find the error. My mistake was that I had left "Data to compare with" empty instead of adding the original value there, too.