Recommend solution for visually indicating invalid input in singleline edit control

I want to be careful about not pressing with my own opinion on such indication, because it depends on individual taste and is a matter of controversies. Perhaps you can choose something appropriate to your application.

Here are some items you would need to take into account:

In some simple cases, you can find a way of making input always valid, for example, if you can filter out unwanted characters. Make sure that you never use more complicated filter (for example, character is filtered out or not, depending on its position in string or other criteria), they would only greatly confuse your user.

It can be the opposite case, when even validation of one control input it impossible, because validation criteria depends on several controls. Such cases take place more often than some thing (and this is the reason why I don't want to pay much attention for the validity indication for a single control; it simply does not solve the problem in general). In this case, focus attention at the point where you need to ultimately use control data. For example, if you need to use data on some button click, enable or disable this button. This will require validation of everything on every change in each of controls, so think about making some universal facility to cover such situations.

—SA