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

regex/compile.go

@@ -14,9 +14,10 @@ var notDotChars []rune
 // the startState of the NFA representation of the regex, and the number of capturing
 // groups in the regex. It also contains the expression string.
 type Reg struct {
-	start     *nfaState
-	numGroups int
-	str       string
+	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
 
 }