sdk: correct langtag check to follow RFC 5646

The langtag check of ITEF BCP 47 format from the aas-core project
was not following the RFC 5646 precisely in two minor important
points.

These deviations are now corrected, so the check strictly follows
the syntax described in the standard.

Author: hpoeche

sdk/basyx/aas/model/base.py

@@ -304,7 +304,7 @@ def _check_language_tag_constraints(cls, ltag: str):
         alphanum = "[a-zA-Z0-9]"
         singleton = "[0-9A-WY-Za-wy-z]"
         extension = f"{singleton}(-({alphanum}){{2,8}})+"
-        extlang = "[a-zA-Z]{3}(-[a-zA-Z]{3}){2}"
+        extlang = "[a-zA-Z]{3}(-[a-zA-Z]{3}){0,2}"
         irregular = (
             "(en-GB-oed|i-ami|i-bnn|i-default|i-enochian|i-hak|"
             "i-klingon|i-lux|i-mingo|i-navajo|i-pwn|i-tao|i-tay|"
@@ -319,7 +319,7 @@ def _check_language_tag_constraints(cls, ltag: str):
         script = "[a-zA-Z]{4}"
         region = "([a-zA-Z]{2}|[0-9]{3})"
         variant = f"(({alphanum}){{5,8}}|[0-9]({alphanum}){{3}})"
-        privateuse = f"[xX](-({alphanum}){{1,8}})+"
+        privateuse = f"[x](-({alphanum}){{1,8}})+"
         langtag = (
             f"{language}(-{script})?(-{region})?(-{variant})*(-{extension})*(-"
             f"{privateuse})?"