Correctly escape strings if needed and check for it

atom.go

@@ -3,6 +3,7 @@ package atomfeed
 import (
 	"mime"
 	"net/url"
+	"regexp"
 	"strings"
 )
 
@@ -13,11 +14,32 @@ type (
 	URI          string
 )
 
-func isValidURL(testURL URI) bool {
-	_, err := url.ParseRequestURI(string(testURL))
+func isValidURL(uri URI) bool {
+	_, err := url.ParseRequestURI(string(uri))
 	return err == nil
 }
 
+func isValidURN(uri URI) bool {
+	pattern := `\A(?i:urn:(?!urn:)(?<nid>[a-z0-9][a-z0-9-]{1,31}):(?<nss>(?:[-a-z0-9()+,.:=@;$_!*'&~\/]|%[0-9a-f]{2})+)(?:\?\+(?<rcomponent>.*?))?(?:\?=(?<qcomponent>.*?))?(?:#(?<fcomponent>.*?))?)\z`
+	return regexp.MustCompile(pattern).MatchString(string(uri))
+}
+
+func isValidURI(uri URI) bool {
+	return isValidURL(uri) || isValidURN(uri)
+}
+
+func isCorrectlyEscaped(text string) bool {
+	relevantEntities := []string{"&amp;", "&lt;", "&gt;", "&quot;", "&apos;"}
+
+	for _, entity := range relevantEntities {
+		if strings.Contains(text, entity) {
+			return false
+		}
+	}
+
+	return true
+}
+
 func isCompositeMediaType(mediaType string) bool {
 	mediaType, _, err := mime.ParseMediaType(mediaType)
 	if err != nil {

category.go

@@ -3,6 +3,7 @@ package atomfeed
 import (
 	"errors"
 	"fmt"
+	"html"
 )
 
 type Category struct {
@@ -22,11 +23,25 @@ func NewCategory(term string) (*Category, error) {
 	return &Category{Term: term, Content: content}, nil
 }
 
+func (c *Category) SetLabel(label string) {
+	c.Label = html.UnescapeString(label)
+}
+
 func (c *Category) Check() error {
 	if c.Term == "" {
 		return errors.New("term attribute of category empty")
 	}
 
+	if c.Scheme != "" {
+		if !isValidURI(c.Scheme) {
+			return fmt.Errorf("scheme attribute of category %v not correctly formatted", c.Scheme)
+		}
+	}
+
+	if !isCorrectlyEscaped(c.Label) {
+		return fmt.Errorf("label attribute of category %v not correctly escaped", c.Label)
+	}
+
 	if c.Content == nil {
 		return errors.New("no content element of category")
 	} else {

outOfLineContent.go

@@ -22,7 +22,7 @@ func newOutOfLineContent(mediaType string, content any) (*OutOfLineContent, erro
 		return nil, fmt.Errorf("content type %T incompatible with out of line content", content)
 	}
 
-	if !isValidURL(content.(URI)) {
+	if !isValidURI(content.(URI)) {
 		return nil, errors.New("content not a valid uri")
 	}
 

person.go

@@ -29,7 +29,7 @@ func (p *Person) Check() error {
 	}
 
 	if p.URI != "" {
-		if !isValidURL(p.URI) {
+		if !isValidURI(p.URI) {
 			return fmt.Errorf("uri element of person %v not correctly formatted", p.Name)
 		}
 	}

plainText.go

@@ -15,6 +15,10 @@ func (p *PlainText) Check() error {
 		return errors.New("type attribute of plain text must be text or html if not omitted")
 	}
 
+	if p.Type == "html" && !isCorrectlyEscaped(p.Text) {
+		return errors.New("text element of plain text not correctly escaped")
+	}
+
 	if p.Text == "" {
 		return errors.New("text element of plain text empty")
 	}

text.go

@@ -15,7 +15,7 @@ func NewText(textType, content string) (Text, error) {
 	case "text", "":
 		return &PlainText{Type: textType, Text: content}, nil
 	case "html":
-		return &PlainText{Type: textType, Text: html.EscapeString(content)}, nil
+		return &PlainText{Type: textType, Text: html.UnescapeString(content)}, nil
 	case "xhtml":
 		return &XHTMLText{
 			Type: textType,