I have had to deprioritize this particular projectfor now but the answers so far contain a lot of useful information and experience which I will need to study. The main point is that people are picking up on my choice of requirements. If they are vague that might be a good thing, seeing as each interpretation of my requirements might elicit more useful information. However I can clarify. My test was rarely that the regular expression should accept "księgowość" but reject "£$%%^&". I was surprised at how hard this was. More generally I was hoping the regular expression would capture "reasonable search terms". As such I would regard a Chinese sentence as valid but an emoticon character as invalid.