slug: Add more validations to IsSlug

The new validations are for empty string, _- as prefix or sufix of the slug and MaxLength validation if setted

slug.go

@@ -138,9 +138,16 @@ func smartTruncate(text string) string {
 
 // IsSlug returns True if provided text does not contain white characters,
 // punctuation, all letters are lower case and only from ASCII range.
-// It could contain `-` and `_`.
+// It could contain `-` and `_` but not at the beginning or end of the text.
+// It should be in range of the MaxLength var if specified.
 // All output from slug.Make(text) should pass this test.
 func IsSlug(text string) bool {
+	if text == "" ||
+		(MaxLength > 0 && len(text) > MaxLength) ||
+		text[0] == '-' || text[0] == '_' ||
+		text[len(text)-1] == '-' || text[len(text)-1] == '_' {
+		return false
+	}
 	for _, c := range text {
 		if (c < 'a' || c > 'z') && c != '-' && c != '_' && (c < '0' || c > '9') {
 			return false

slug_test.go

@@ -207,6 +207,7 @@ func TestSlugMakeSmartTruncate(t *testing.T) {
 }
 
 func TestIsSlug(t *testing.T) {
+	MaxLength = 0
 	type args struct {
 		text string
 	}
@@ -219,8 +220,13 @@ func TestIsSlug(t *testing.T) {
 		{"with -", args{"some-more"}, true},
 		{"with _", args{"some_more"}, true},
 		{"with numbers", args{"number-2"}, true},
+		{"empty string", args{""}, false},
 		{"upper case", args{"Some-more"}, false},
 		{"space", args{"some more"}, false},
+		{"starts with '-'", args{"-some"}, false},
+		{"ends with '-'", args{"some-"}, false},
+		{"starts with '_'", args{"_some"}, false},
+		{"ends with '_'", args{"some_"}, false},
 		{"outside ASCII", args{"Dobrosław Żybort"}, false},
 		{"outside ASCII –", args{"2000–2013"}, false},
 		{"smile ☺", args{"smile ☺"}, false},
@@ -232,6 +238,14 @@ func TestIsSlug(t *testing.T) {
 			}
 		})
 	}
+
+	t.Run("MaxLength", func(t *testing.T) {
+		MaxLength = 4
+		if got := IsSlug("012345"); got != false {
+			t.Errorf("IsSlug() = %v, want %v", got, false)
+		}
+		MaxLength = 0
+	})
 }
 
 func BenchmarkMakeShortAscii(b *testing.B) {