Basic grep working, no recursive though.

grep/.gitignore

@@ -0,0 +1,2 @@
+grep
+grep.exe

grep/Makefile

@@ -0,0 +1,4 @@
+
+
+build:
+	go build .

grep/go.mod

@@ -0,0 +1,3 @@
+module lcthw.dev/go/go-coreutils/grep
+
+go 1.25.3

grep/main.go

@@ -0,0 +1,52 @@
+package main
+
+import (
+  "fmt"
+  "regexp"
+  "os"
+  "bufio"
+  "flag"
+  "log"
+  "io"
+)
+
+func ScanInput(input io.Reader, exp string, prefix string) {
+	scan := bufio.NewScanner(input)
+	re, err := regexp.Compile(exp)
+
+	if err != nil { log.Fatal(err) }
+
+	for scan.Scan() {
+		line := scan.Text()
+
+		if re.MatchString(line) {
+			if prefix != "" {
+				fmt.Print(prefix, ": ")
+			}
+
+			fmt.Println(line)
+		}
+	}
+}
+
+func main() {
+	flag.Parse()
+
+	args := flag.Args()
+
+	if len(args) == 0 {
+		log.Fatal("USAGE: grep <regex> [files...]")
+	} else if len(args) == 1 {
+		ScanInput(os.Stdin, args[0], "")
+	} else {
+		exp := args[0]
+		files := args[1:]
+
+		for _, file := range files {
+			input, err := os.Open(file)
+			if err != nil { log.Fatal(err) }
+
+			ScanInput(input, exp, file)
+		}
+	}
+}