init

LICENSE

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+NIRVÁNA LICENSE
+
+As this software enters the public domain,
+it achieves a state of nirvána - complete liberation
+from the cycle of intellectual property constraints.

README.md

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+# is-valid-domain
+
+A CLI tool to validate domain names using the public suffix list from `golang.org/x/net/publicsuffix`.
+
+## Usage
+
+```bash
+./is-valid-domain <domain>
+```
+
+## Examples
+
+```bash
+./is-valid-domain 029.cyou          # Output: valid
+./is-valid-domain siliconpin.co.in   # Output: valid
+./is-valid-domain co.siliconpin.in   # Output: subdomain
+./is-valid-domain invalid..domain    # Output: invalid
+```
+
+## Output Categories
+
+- **valid** - The domain is a registrable domain (effective TLD+1)
+- **subdomain** - The domain is a subdomain of a registrable domain
+- **invalid** - The domain is malformed or not a valid domain name
+
+## Installation
+
+```bash
+go build -o is-valid-domain main.go
+```
+
+## How it Works
+
+The tool uses `publicsuffix.EffectiveTLDPlusOne()` to determine the registrable domain:
+
+1. If the input domain exactly matches the effective TLD+1 → **valid**
+2. If the input domain is longer than the effective TLD+1 → **subdomain**
+3. If there's an error parsing the domain → **invalid**
+
+This leverages the official public suffix list to correctly handle complex TLD structures like `.co.in`, `.co.uk`, etc.

go.mod

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+module is-valid-domain
+
+go 1.25.5
+
+require golang.org/x/net v0.51.0 // indirect

go.sum

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+golang.org/x/net v0.51.0 h1:94R/GTO7mt3/4wIKpcR5gkGmRLOuE/2hNGeWq/GBIFo=
+golang.org/x/net v0.51.0/go.mod h1:aamm+2QF5ogm02fjy5Bb7CQ0WMt1/WVM7FtyaTLlA9Y=

main.go

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+package main
+
+import (
+	"fmt"
+	"os"
+	"strings"
+
+	"golang.org/x/net/publicsuffix"
+)
+
+func main() {
+	if len(os.Args) != 2 {
+		fmt.Fprintf(os.Stderr, "Usage: %s <domain>\n", os.Args[0])
+		os.Exit(1)
+	}
+
+	domain := strings.TrimSpace(os.Args[1])
+	if domain == "" {
+		fmt.Println("invalid")
+		os.Exit(1)
+	}
+
+	// Get the effective TLD+1 (registrable domain)
+	etld1, err := publicsuffix.EffectiveTLDPlusOne(domain)
+	if err != nil {
+		fmt.Println("invalid")
+		os.Exit(1)
+	}
+
+	// If the domain is exactly the effective TLD+1, it's a valid domain
+	// If it's longer, it's a subdomain
+	if domain == etld1 {
+		fmt.Println("valid")
+	} else {
+		fmt.Println("subdomain")
+	}
+}