replace matches in input if input pattern does not match a whole line (grep functionality)

main.go

@@ -10,6 +10,8 @@ import (
 	"strconv"
 	"strings"
 	"unicode/utf8"
+
+	"github.com/fatih/color"
 )
 
 var (
@@ -58,10 +60,37 @@ func main() {
 		panic(err)
 	}
 
-	escapedOutput := EscSeqReplacer.Replace(*output)
+	matchesWholeLine := strings.HasPrefix(*input, "^") && strings.HasSuffix(*input, "$")
 
+	if matchesWholeLine {
+		escapedOutput := EscSeqReplacer.Replace(*output)
+		handleWholeLineRegex(pattern, escapedOutput)
+	} else {
+		handleInlineRegex(pattern)
+	}
+}
+
+// handleWholeLineRegex is using input pattern ot replace
+// placeholders in output pattern with the given subgroups
+func handleWholeLineRegex(pattern *regexp.Regexp, output string) {
 	for line := range readLines(os.Stdin) {
 		matches := pattern.FindStringSubmatch(line)
+		if len(matches) == 0 {
+			if *keepUnmatched {
+				fmt.Println(line)
+			}
+			continue
+		}
+		fmt.Println(replaceVars(output, matches...))
+	}
+}
+
+// handleInlineRegex is using input pattern
+// and color-codes all matches within input
+func handleInlineRegex(pattern *regexp.Regexp) {
+	c := color.New(color.FgRed, color.Bold)
+	for line := range readLines(os.Stdin) {
+		matches := pattern.FindAllStringIndex(line, -1)
 
 		if len(matches) == 0 {
 			if *keepUnmatched {
@@ -70,7 +99,26 @@ func main() {
 			continue
 		}
 
-		fmt.Println(replaceVars(escapedOutput, matches...))
+		runes := []rune(line)
+		b := new(strings.Builder)
+		nextMatch := 0
+		currentIndex := 0
+		for currentRune := 0; currentRune < len(runes); currentRune++ {
+			if nextMatch >= len(matches) || currentIndex < matches[nextMatch][0] {
+				// handle next rune
+				b.WriteRune(runes[currentRune])
+				currentIndex += utf8.RuneLen(runes[currentRune])
+			} else {
+				// handle next match
+				match := line[matches[nextMatch][0]:matches[nextMatch][1]]
+				b.WriteString(c.Sprint(match))
+				currentIndex += len(match)
+				currentRune += utf8.RuneCountInString(match) - 1
+				nextMatch++
+			}
+		}
+
+		fmt.Println(b.String())
 	}
 }