Added string field to Reg, that contains the expression string; wrote method to return the string

regex/compile.go

@@ -12,18 +12,24 @@ var notDotChars []rune
 
 // A Reg represents the result of compiling a regular expression. It contains
 // the startState of the NFA representation of the regex, and the number of capturing
-// groups in the regex.
+// groups in the regex. It also contains the expression string.
 type Reg struct {
 	start     *nfaState
 	numGroups int
+	str       string
 }
 
-// numSubexp eturns the number of sub-expressions in the given [Reg]. This is equivalent
+// NumSubexp returns the number of sub-expressions in the given [Reg]. This is equivalent
 // to the number of capturing groups.
 func (r Reg) NumSubexp() int {
 	return r.numGroups
 }
 
+// String returns the string used to compile the expression.
+func (r Reg) String() string {
+	return r.str
+}
+
 const concatRune rune = 0xF0001
 
 // Flags for shuntingYard - control its behavior
@@ -1128,7 +1134,8 @@ func thompson(re []postfixNode) (Reg, error) {
 
 	concatenate(nfa[0], &lastState)
 
-	return Reg{nfa[0], numGroups}, nil
+	// The string is empty here, because we add it in Compile()
+	return Reg{nfa[0], numGroups, ""}, nil
 
 }
 
@@ -1146,6 +1153,7 @@ func Compile(re string, flags ...ReFlag) (Reg, error) {
 	if err != nil {
 		return Reg{}, fmt.Errorf("error compiling regex: %w", err)
 	}
+	reg.str = re
 	return reg, nil
 }