Add IPv4 string validation

modules/core/shared/src/main/scala/eu/timepit/refined/string.scala

@@ -15,6 +15,9 @@ object string extends StringValidate with StringInference {
   /** Predicate that checks if a `String` ends with the suffix `S`. */
   final case class EndsWith[S](s: S)
 
+  /** Predicate that checks if a `String` is a valid IPv4 */
+  final case class IPv4()
+
   /** Predicate that checks if a `String` matches the regular expression `S`. */
   final case class MatchesRegex[S](s: S)
 
@@ -40,14 +43,34 @@ object string extends StringValidate with StringInference {
   final case class XPath()
 }
 
-private[refined] trait StringValidate {
+object Predicates {
+  object IPv4 {
+    val regex = "^(\\d{1,3})\\.(\\d{1,3})\\.(\\d{1,3})\\.(\\d{1,3})$".r.pattern
+    val maxOctet = 255
+    val predicate: String => Boolean = s => {
+      val matcher = regex.matcher(s)
+      matcher.find() && matcher.matches() && {
+        val octet1 = matcher.group(1).toInt
+        val octet2 = matcher.group(2).toInt
+        val octet3 = matcher.group(3).toInt
+        val octet4 = matcher.group(4).toInt
+
+        (octet1 <= maxOctet) && (octet2 <= maxOctet) && (octet3 <= maxOctet) && (octet4 <= maxOctet)
+      }
+    }
+  }
+}
 
+private[refined] trait StringValidate {
   implicit def endsWithValidate[S <: String](
       implicit ws: Witness.Aux[S]): Validate.Plain[String, EndsWith[S]] =
     Validate.fromPredicate(_.endsWith(ws.value),
                            t => s""""$t".endsWith("${ws.value}")""",
                            EndsWith(ws.value))
 
+  implicit def ipv4Validate[S <: String]: Validate.Plain[String, IPv4] =
+    Validate.fromPredicate(Predicates.IPv4.predicate, t => s"${t} is a valid IPv4", IPv4())
+
   implicit def matchesRegexValidate[S <: String](
       implicit ws: Witness.Aux[S]): Validate.Plain[String, MatchesRegex[S]] =
     Validate.fromPredicate(_.matches(ws.value),

modules/core/shared/src/test/scala/eu/timepit/refined/StringValidateSpec.scala

@@ -69,4 +69,16 @@ class StringValidateSpec extends Properties("StringValidate") {
   property("Uuid.showResult.Failed") = secure {
     showResult[Uuid]("whops") ?= "Uuid predicate failed: Invalid UUID string: whops"
   }
+
+  property("IPv4.isValid") = secure {
+    isValid[IPv4]("10.0.0.1")
+  }
+
+  property("IPv4.showResult.InvalidOctet") = secure {
+    showResult[IPv4]("10.0.256.1") ?= "Predicate failed: 10.0.256.1 is a valid IPv4."
+  }
+
+  property("IPv4.showResult.Failed") = secure {
+    showResult[IPv4]("::1") ?= "Predicate failed: ::1 is a valid IPv4."
+  }
 }