Started working on file arguments - stdin is used if arg is "-"

cmd/main.go

@@ -70,12 +70,24 @@ func main() {
 	}
 	var re string
 	re = flag.Args()[0]
+	var inputFile *os.File
+	if len(flag.Args()) == 1 || flag.Args()[1] == "-" { // Either no file argument, or file argument is "-"
+		inputFile = os.Stdin
+	} else {
+		var err error
+		inputFile, err = os.Open(flag.Args()[1])
+		if err != nil {
+			fmt.Printf("%s: No such file or directory\n", flag.Args()[1])
+			os.Exit(2)
+		}
+	}
+
 	var test_str string
 	var err error
 	var linesRead bool // Whether or not we have read the lines in the file
 	lineNum := 0       // Current line number
 	// Create reader for stdin and writer for stdout
-	reader := bufio.NewReader(os.Stdin)
+	reader := bufio.NewReader(inputFile)
 	out := bufio.NewWriter(os.Stdout)
 
 	regComp, err := reg.Compile(re, flagsToCompile...)