Added support for -l : only print lines with at least one match (or with exactly 0 matches, if -v is enabled

main.go

@@ -375,9 +375,15 @@ func main() {
 	// This flag has two 'modes':
 	// 1. Without '-v': Prints only matches. Prints a newline after every match.
 	// 2. With '-v': Substitutes all matches with empty string.
-	onlyFlag := flag.Bool("o", false, "Print only colored content.")
+	onlyFlag := flag.Bool("o", false, "Print only colored content. Overrides -l.")
+	lineFlag := flag.Bool("l", false, "Only print lines with a match (or with no matches, if -v is enabled")
 	flag.Parse()
 
+	// -l and -o are mutually exclusive: -o overrides -l
+	if *onlyFlag {
+		*lineFlag = false
+	}
+
 	// Process:
 	// 1. Convert regex into postfix notation (Shunting-Yard algorithm)
 	// 		a. Add explicit concatenation operators to facilitate this
@@ -392,7 +398,7 @@ func main() {
 	re = flag.Args()[0]
 	var test_str string
 	var err error
-	// Create reader for stdin and writer for stdout
+	// Create reader for stdin and writer for stdout // End index is one more than last index of match
 	reader := bufio.NewReader(os.Stdin)
 	out := bufio.NewWriter(os.Stdout)
 
@@ -419,11 +425,21 @@ func main() {
 			indicesToPrint.add(setDifference(genRange(0, len(test_str)), oldIndices)...)
 
 		}
+		// If lineFlag is enabled, we should only print something if:
+		// 		a. We are not inverting, and have at least one match on the current line
+		// 		OR
+		// 		b. We are inverting, and have no matches at all on the current line.
+		// This checks for the inverse, and continues if it is true.
+		if *lineFlag {
+			if !(*invertFlag) && len(matchIndices) == 0 || *invertFlag && len(matchIndices) > 0 {
+				continue
+			}
+		}
 		for i, c := range test_str {
 			if indicesToPrint.contains(i) {
 				color.New(color.FgRed).Fprintf(out, "%c", c)
-				// Newline after every match - only if -v is disabled.
-				if !(*invertFlag) {
+				// Newline after every match - only if -o is enabled and -v is disabled.
+				if *onlyFlag && !(*invertFlag) {
 					for _, idx := range matchIndices {
 						if i+1 == idx.endIdx { // End index is one more than last index of match
 							fmt.Fprintf(out, "\n")