Add .gitignore and main.go with input parsing logic

.gitignore

@@ -0,0 +1 @@
+input.txt

day1/example.txt

@@ -0,0 +1,4 @@
+1abc2
+pqr3stu8vwx
+a1b2c3d4e5f
+treb7uchet

day1/main.go

@@ -0,0 +1,42 @@
+package main
+
+import (
+	"io"
+	"os"
+	"regexp"
+	"strconv"
+	"strings"
+)
+
+func readInput() string {
+	stdin, err := io.ReadAll(os.Stdin)
+
+	if err != nil {
+		panic(err)
+	}
+	str := string(stdin)
+	return str
+}
+
+func main() {
+	input := readInput()
+	lines := strings.Split(input, "\n")
+	re := regexp.MustCompile(`\d`)
+	var answer int
+	for _, l := range lines {
+		matches := re.FindAllString(l, -1)
+		matches_len := len(matches)
+		if matches_len == 0 {
+			continue
+		}
+		first := matches[0]
+		last := matches[matches_len-1]
+		num_str := first + last
+		num, err := strconv.Atoi(num_str)
+		if err != nil {
+			panic(err)
+		}
+		answer = answer + num
+	}
+	println(answer)
+}