Added field to Reg, denoting if we prefer longest match (POSIX style) or not (perl style)

regex/compile.go

@@ -17,6 +17,7 @@ type Reg struct {
 	start         *nfaState
 	numGroups     int
 	str           string
+	preferLongest bool
 }
 
 // NumSubexp returns the number of sub-expressions in the given [Reg]. This is equivalent
@@ -30,6 +31,10 @@ func (r Reg) String() string {
 	return r.str
 }
 
+func (r Reg) Longest() {
+	r.preferLongest = true
+}
+
 const concatRune rune = 0xF0001
 
 // Flags for shuntingYard - control its behavior
@@ -1135,7 +1140,7 @@ func thompson(re []postfixNode) (Reg, error) {
 	concatenate(nfa[0], &lastState)
 
 	// The string is empty here, because we add it in Compile()
-	return Reg{nfa[0], numGroups, ""}, nil
+	return Reg{nfa[0], numGroups, "", false}, nil
 
 }