gh-139489: Add xml.sax.utils.is_valid_name()

It allows to check whether a string can be used as an element or attribute
name in XML.

Author: serhiy-storchaka

Doc/library/xml.sax.utils.rst

@@ -59,6 +59,20 @@ or as base classes.
    using the reference concrete syntax.
 
 
+.. function:: is_valid_name(name)
+
+   Return ``True`` if the string is a valid element or attribute name,
+   ``False`` otherwise.
+
+   Almost all characters are permitted in names, except control characters and
+   those which either are or reasonably could be used as delimiters.
+   Characters like ":", "-", ".", "_", and "·" are permitted, but "<", "/",
+   "!", "?", and "=" are forbidden.
+   The name cannot start with a digit or a character like "-", ".", and "·".
+
+   ..versionadded:: next
+
+
 .. class:: XMLGenerator(out=None, encoding='iso-8859-1', short_empty_elements=False)
 
    This class implements the :class:`~xml.sax.handler.ContentHandler` interface

Doc/whatsnew/3.15.rst

@@ -580,6 +580,14 @@ xml.parsers.expat
   .. _billion laughs: https://en.wikipedia.org/wiki/Billion_laughs_attack
 
 
+xml.sax.utils
+-------------
+
+* Add the :func:`~xml.sax.utils.is_valid_name` function, which allows to check
+  whether a string can be used as an element or attribute name in XML.
+  (Contributed by Serhiy Storchaka in :gh:`139489`.)
+
+
 zlib
 ----
 

Lib/test/test_sax.py

@@ -9,8 +9,9 @@
 except SAXReaderNotAvailable:
     # don't try to test this module if we cannot create a parser
     raise unittest.SkipTest("no XML parsers available")
-from xml.sax.saxutils import XMLGenerator, escape, unescape, quoteattr, \
-                             XMLFilterBase, prepare_input_source
+from xml.sax.saxutils import (XMLGenerator, escape, unescape, quoteattr,
+                              is_valid_name,
+                              XMLFilterBase, prepare_input_source)
 from xml.sax.expatreader import create_parser
 from xml.sax.handler import (feature_namespaces, feature_external_ges,
                              LexicalHandler)
@@ -343,6 +344,23 @@ def test_single_double_quoteattr(self):
         self.assertEqual(quoteattr("Includes 'single' and \"double\" quotes"),
                          "\"Includes 'single' and "double" quotes\"")
 
+    def test_is_valid_name(self):
+        self.assertFalse(is_valid_name(''))
+        self.assertTrue(is_valid_name('name'))
+        self.assertTrue(is_valid_name('NAME'))
+        self.assertTrue(is_valid_name('name0:-._·'))
+        self.assertTrue(is_valid_name('_'))
+        self.assertTrue(is_valid_name(':'))
+        self.assertTrue(is_valid_name('Ñàḿĕ'))
+        self.assertTrue(is_valid_name('\U000EFFFF'))
+        self.assertFalse(is_valid_name('0'))
+        self.assertFalse(is_valid_name('-'))
+        self.assertFalse(is_valid_name('.'))
+        self.assertFalse(is_valid_name('·'))
+        self.assertFalse(is_valid_name('na me'))
+        for c in '<>/!?=\x00\x01\x7f\ud800\udfff\ufffe\uffff\U000F0000':
+            self.assertFalse(is_valid_name('name' + c))
+
     # ===== make_parser
     def test_make_parser(self):
         # Creating a parser should succeed - it should fall back

Lib/xml/sax/saxutils.py

@@ -3,9 +3,12 @@
 convenience of application and driver writers.
 """
 
-import os, urllib.parse, urllib.request
-import io
 import codecs
+import io
+import os
+import re
+import urllib.parse
+import urllib.request
 from . import handler
 from . import xmlreader
 
@@ -67,6 +70,29 @@ def quoteattr(data, entities={}):
         data = '"%s"' % data
     return data
 
+def is_valid_name(name):
+    """Test whether a string is a valid element or attribute name."""
+    # https://www.w3.org/TR/xml/#NT-Name
+    return re.fullmatch(
+        # NameStartChar
+        '['
+            ':A-Z_a-z'
+            '\xC0-\xD6\xD8-\xF6\xF8-\u02FF\u0370-\u037D\u037F-\u1FFF'
+            '\u200C\u200D'
+            '\u2070-\u218F\u2C00-\u2FEF\u3001-\uD7FF'
+            '\uF900-\uFDCF\uFDF0-\uFFFD\U00010000-\U000EFFFF'
+        ']'
+        # NameChar
+        '['
+            r'\-.0-9:A-Z_a-z'
+            '\xB7'
+            '\xC0-\xD6\xD8-\xF6\xF8-\u037D\u037F-\u1FFF'
+            '\u200C\u200D\u203F\u2040'
+            '\u2070-\u218F\u2C00-\u2FEF\u3001-\uD7FF'
+            '\uF900-\uFDCF\uFDF0-\uFFFD\U00010000-\U000EFFFF'
+        ']*',
+        name) is not None
+
 
 def _gettextwriter(out, encoding):
     if out is None:

Misc/NEWS.d/next/Library/2025-10-08-15-36-00.gh-issue-139489.W46tvn.rst

@@ -0,0 +1,2 @@
+Add the :func:`~xml.sax.utils.is_valid_name` function, which allows to check
+whether a string can be used as an element or attribute name in XML.