Added support for -v flag, to invert which values are printed in color. Also got rid of unecessary 'else' clause

main.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"bufio"
+	"flag"
 	"fmt"
 	"io"
 	"os"
@@ -370,18 +371,22 @@ func thompson(re []postfixNode) *State {
 }
 
 func main() {
+	invertFlag := flag.Bool("v", false, "Invert match.")
+	//onlyFlag := flag.Bool("o", false, "Print only colored content.")
+	flag.Parse()
+
 	// Process:
 	// 1. Convert regex into postfix notation (Shunting-Yard algorithm)
 	// 		a. Add explicit concatenation operators to facilitate this
 	// 2. Build NFA from postfix representation (Thompson's algorithm)
 	// 3. Run the string against the NFA
 
-	if len(os.Args) != 2 {
+	if len(flag.Args()) != 1 { // flag.Args() also strips out program name
 		fmt.Println("ERROR: Missing cmdline args")
 		os.Exit(22)
 	}
 	var re string
-	re = os.Args[1]
+	re = flag.Args()[0]
 	var test_str string
 	var err error
 	// Create reader for stdin and writer for stdout
@@ -399,16 +404,24 @@ func main() {
 		for _, idx := range matchIndices {
 			indicesToPrint.add(genRange(idx.startIdx, idx.endIdx)...)
 		}
-		if len(matchIndices) > 0 {
-			for i, c := range test_str {
-				if indicesToPrint.contains(i) {
-					color.New(color.FgRed).Fprintf(out, "%c", c)
-				} else {
-					fmt.Fprintf(out, "%c", c)
-				}
+		// If we are inverting, then we should print the indices which _didn't_ match
+		// in color.
+		if *invertFlag {
+			oldIndices := indicesToPrint.values()
+			indicesToPrint = new_uniq_arr[int]()
+			// Explanation:
+			// Find all numbers from 0 to len(test_str) that are NOT in oldIndices.
+			// These are the values we want to print, now that we have inverted the match.
+			// Re-initialize indicesToPrint and add all of these values to it.
+			indicesToPrint.add(setDifference(genRange(0, len(test_str)), oldIndices)...)
+
+		}
+		for i, c := range test_str {
+			if indicesToPrint.contains(i) {
+				color.New(color.FgRed).Fprintf(out, "%c", c)
+			} else {
+				fmt.Fprintf(out, "%c", c)
 			}
-		} else {
-			fmt.Fprint(out, test_str)
 		}
 		err = out.Flush()
 		if err != nil {