However, I don't see a webuser entering in config information in a script to modify say a squid config file or a squidGuard config file which is unforgiving to say the least. I would love to be able to have the time to write a script to do just that. If I want to allow modifications to a system configuration file then I have to abide by the configuration rules of the application/service I am changing.

For me to see a "smtwhfa" in a posting I immediately think about a script to control a configuration file. I could never see asking "any" user to come up with those abreviations for the weekdays. Having been around systems for a while the above mentioned letters are very familiar. I am sure the OP has considered allowing a user to enter things such as Monday, Tuesday, Wednesday etc. then converting it to m,t,w, or even mtw... But I am not going to assume that his request is for a web application.

If I am going to update a config file that has the ability to accept "smtwhfa" (or whatever the last letter may be) I have to write the program to be able to accept that and maybe s-f, or Mon-Fri, or (Monday, Wednesday-Saturday), etc, etc, etc. This list of and combination of could go on forever.

I am not too sure that this posting is for a web application for any "user" to be disturbed by the output as much as it is for an incredible regex problem that a lot of people (including myself) have encountered. As for rules to make sure that a user enters in information that you can easily process I don't see that as all bad. I use rules more for myself than I do the user. Many times I have been shocked as an end user entered something into an online form that without some sort of rules resulted in catastrophe. Rules can be a good thing as it allows us not to get caught with our pants down. You can't under estimate the user..