not “foob” is the challenge. “foob[aeiou]r” is it, that expands to
foobar
foober
foobir
foobor
foobur
which requires five rules instead one in emClient. This gets confusing very fast and makes it really hard to keep the overview of all rules.
Especially the scanning for SPAM or JUNK requires a more flexible way – only header is not enough, because spammer use valid adresses nowadays, the body-part is relevant.
And there often an intentionally typo (nude, nuude, nUde, nu-de), that can not be catched reliably with simple strings. But a regular rule “n(.?[Uu?].?)de” catches them and the other typos similar to that.
Because of my experience that whitelisting does not work reliably in emClient I do not trust in working with header only, it is risky.
Whatever field you are searching in, fieldname: foob will give you all the options you have listed. Basically it is *foob*. I think that the search foob[aeiou]r is highly specialized. Out of interest, what email client do you use that you can use that format?
As you said: it is the same as *foob*. This means, it will find blafoob, blifoob, foobzr, foobr … – and any other combination, additionally to the only I want (foobar, foober, foobir, boobor or foobur). And match only small letters, please.
I am talking about advanced, professional search – searching for a simple ( trunk of a ) string is basic – and this is all emClient offers. The reduction to as few as possible matches is necessary in > 100.000 mails that I regularily have to scan for something. Because even these can be more than 100, that have to be searched for “the one”.
This search is available eg in The Bat, afaik Thunderbird, Enigmail, and others (I do not use them, therefore I am not an expert in functionality overview).
Note: My first comment was wrong, I read “whicht em Client is offering that” – were the answer (currently and regretably) is “none”.