Update .gitignore, main.go

.gitignore

@@ -0,0 +1 @@
+format

main.go

@@ -0,0 +1,83 @@
+package main
+
+import (
+	"bufio"
+	"flag"
+	"fmt"
+	"os"
+	"regexp"
+	"strconv"
+	"strings"
+)
+
+var (
+	// regex with sub groups
+	input = flag.String("i", "^.*?$", "input pattern")
+
+	// format string with {0} as placeholders
+	// {0} always matches the whole line
+	// {1} and onwards match their respective sub groups
+	// You can optionally specify a printf-syntax for formatting like this: {1:%s} or {1:%02d}
+	// printf-syntax is currently supported for: strings, floats, integers
+	output = flag.String("o", "{0}", "output pattern")
+
+	// don't ignore lines which do not match against input.
+	// they will be copied without any changes
+	keepUnmatched = flag.Bool("k", false, "keep unmatched lines")
+
+	replacePattern = regexp.MustCompile(`\{(\d+)(?::(.*?))?\}`)
+)
+
+func main() {
+	flag.Parse()
+
+	pattern, err := regexp.Compile(*input)
+	if err != nil {
+		panic(err)
+	}
+
+	r := bufio.NewReader(os.Stdin)
+	for line, err := r.ReadString('\n'); err == nil; line, err = r.ReadString('\n') {
+		line = line[:len(line)-1]
+		matches := pattern.FindStringSubmatch(line)
+
+		fmt.Println(matches)
+
+		if len(matches) == 0 {
+			if *keepUnmatched {
+				fmt.Println(line)
+			}
+			continue
+		}
+
+		fmt.Println(replaceVars(*output, matches...))
+	}
+}
+
+func replaceVars(format string, vars ...string) string {
+	replacements := replacePattern.FindAllStringSubmatch(format, -1)
+
+	for _, replacement := range replacements {
+		rplStr := replacement[0]
+		varIndex, _ := strconv.Atoi(replacement[1])
+		rplFmt := replacement[2]
+
+		// default format if not specified by user
+		if rplFmt == "" {
+			rplFmt = "%s"
+		}
+
+		if strings.HasSuffix(rplFmt, "d") { // replace integers
+			value, _ := strconv.ParseInt(vars[varIndex], 10, 64)
+			format = strings.Replace(format, rplStr, fmt.Sprintf(rplFmt, value), 1)
+		} else if strings.HasSuffix(rplFmt, "f") { // replace floats
+			value, _ := strconv.ParseFloat(vars[varIndex], 64)
+			format = strings.Replace(format, rplStr, fmt.Sprintf(rplFmt, value), 1)
+		} else { // replace strings
+			format = strings.Replace(format, rplStr, fmt.Sprintf(rplFmt, vars[varIndex]), 1)
+		}
+
+	}
+
+	return format
+}