all: allow colon ':' and period '.' on the ID

According to XML spec [1], the colon is allowed as the first and the next
character.  While period is only allowed on the next characters.

[1] https://www.w3.org/TR/REC-xml/#NT-Name

1 files changed, 3 insertions, 3 deletions

diff --git a/parser.go b/parser.go
index fdc6daa..8b0b8d5 100644
--- a/parser.go
+++ b/parser.go
@@ -497,13 +497,13 @@ func isTitle(line []byte) bool {
 func isValidID(id []byte) bool {
 	for x, r := range string(id) {
 		if x == 0 {
-			if !(r == '-' || r == '_' || unicode.IsLetter(r)) {
+			if !(r == ':' || r == '-' || r == '_' || unicode.IsLetter(r)) {
 				return false
 			}
 			continue
 		}
-		if r == '-' || r == '_' || unicode.IsLetter(r) ||
-			unicode.IsDigit(r) {
+		if r == ':' || r == '-' || r == '_' || r == '.' ||
+			unicode.IsLetter(r) || unicode.IsDigit(r) {
 			continue
 		}
 		return false