Some by "regex" mean "globs" and you can just use "*" and "?" for stand-ins for some number of characters.

Some allow "|", some allow backrefs, some allow "()", or require them escaped with \, or allow them but not with * after. Some are case insensitive, some not. Some allow "{0-5}", some allow "[0-9]", some have handy things like "\w".

It's just the guessing game of exactly what they want. It should be required that each regex box has an example next to it using as many allowed features as possible.