Added substitute flag - substitute matched text with given text

main.go

@@ -507,6 +507,7 @@ func main() {
 	multiLineFlag := flag.Bool("t", false, "Multi-line mode. Treats newline just like any character.")
 	printMatchesFlag := flag.Bool("p", false, "Prints start and end index of each match. Can only be used with '-t' for multi-line mode.")
 	caseInsensitiveFlag = flag.Bool("i", false, "Case-insensitive. Disregard the case of all characters.")
+	substituteText := flag.String("s", "", "Substitute the contents  of each match with the given string. Overrides -o and -v")
 	flag.Parse()
 
 	// In multi-line mode, 'dot' metacharacter also matches newline
@@ -519,6 +520,13 @@ func main() {
 	if *onlyFlag {
 		*lineFlag = false
 	}
+	// Check if substitute text has been enabled
+	substituteFlagEnabled := false
+	flag.Visit(func(f *flag.Flag) {
+		if f.Name == "s" {
+			substituteFlagEnabled = true
+		}
+	})
 
 	// Process:
 	// 1. Convert regex into postfix notation (Shunting-Yard algorithm)
@@ -607,6 +615,29 @@ func main() {
 				continue
 			}
 		}
+
+		// If we are substituting, we need a different behavior, as follows:
+		// For every character in the test string:
+		// 		1. Check if the index is the start of any matchIndex
+		// 		2. If so, print the substitute text, and set our index to
+		//			the corresponding end index.
+		// 		3. If not, just print the character.
+		if substituteFlagEnabled {
+			for i := range test_runes {
+				inMatchIndex := false
+				for _, idx := range matchIndices {
+					if i == idx.startIdx {
+						fmt.Fprintf(out, "%s", *substituteText)
+						i = idx.endIdx
+						inMatchIndex = true
+						break
+					}
+				}
+				if !inMatchIndex {
+					fmt.Fprintf(out, "%c", test_runes[i])
+				}
+			}
+		} else {
 			for i, c := range test_runes {
 				if indicesToPrint.contains(i) {
 					color.New(color.FgRed).Fprintf(out, "%c", c)
@@ -625,6 +656,7 @@ func main() {
 					}
 				}
 			}
+		}
 		err = out.Flush()
 		if err != nil {
 			panic(err)