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

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

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

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;

q(?<number>d+)((?<monday>(?<!MD*)M)|(?<tuesday>(?<!T(?!H)D*)T(?!H))|(?<wednesday>(?<!WD*)W)|(?<thursday>(?<!THD*)TH)|(?<friday>(?<!FD*)F)|(?<saturday>(?<!S(?!U)D*)S(?!U))|(?<sunday>(?<!SUD*)SU))+

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

q(d*)(TH|SU|[MTWFS])+(?<=qd*M?T?W?(?:TH)?F?S?(?:SU)?)