Match exactly one occurrence with regex

Question

Consider M,T,W,TH,F,S,SU are days of week.

I have regex which is working well except for one scenario when there is no sequence of weekdays, i.e. there is no M, T , W , TH , F , S , SU at the expected location inside the string.

For example, q10MT is valid but q10HT is invalid.

Below is my expression:

string expression = "q(\\d*)(M)?(T(?!H))?(W)?(TH)?(F)?(S(?!U))?(SU)?";

In case of q10MT, the output is q10MT which is correct, but in case of q10HT the output is q10 which is incorrect, my regex should return no value or empty string when there is no match.

What changes do I need to make in order to achieve this?

Answer 1

You can achieve it with a positive look-ahead:

q(\\d*)(?=(?:M|T(?!H)|W|TH|F|S(?!U)|SU))(M)?(T(?!H))?(W)?(TH)?(F)?(S(?!U))?(SU)?

Or, as @Taemyr noted, a shorter equivalent

q(\\d*)(?=(?:M|TH?|W|TH|F|SU?))(M)?(T(?!H))?(W)?(TH)?(F)?(S(?!U))?(SU)?

Here is a demo

The (?=(?:M|TH?|W|F|SU?)) look-ahead makes sure there is at least one required value from the alternation list you have after the look-ahead.

C# regex usage:

var rx = new Regex(@"q(\d*)(?=(?:M|TH?|W|TH|F|SU?))(M)?(T(?!H))?(W)?(TH)?(F)?(S(?!U))?(SU)?");
var result = rx.Match("q10MSUT").Value;

Result:

Answer 2

What about the following:

q(\d*)(M|TH?|W|F|SU?)+

See demo with some examples on matches and no-matches. The key change in this regexp is that this one uses the + to require at least one match.

Be aware that this solution doesn't demand the days to be in order, and allows skipping of days specified in comments not to matter.

Edit: OP says in comments that he requires only one match for each day, which this solution doesn't account for.