Implement MSC4133 to support custom profile fields. (#17488)

Implementation of
[MSC4133](https://github.com/matrix-org/matrix-spec-proposals/pull/4133)
to support custom profile fields. It is behind an experimental flag and
includes tests.


### Pull Request Checklist

<!-- Please read
https://element-hq.github.io/synapse/latest/development/contributing_guide.html
before submitting your pull request -->

* [x] Pull request is based on the develop branch
* [x] Pull request includes a [changelog
file](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#changelog).
The entry should:
- Be a short description of your change which makes sense to users.
"Fixed a bug that prevented receiving messages from other servers."
instead of "Moved X method from `EventStore` to `EventWorkerStore`.".
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* [x] [Code
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---------

Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>

1 files changed, 12 insertions, 0 deletions

diff --git a/synapse/util/stringutils.py b/synapse/util/stringutils.py

index 13ff54b669..32b5bc00c9 100644
--- a/synapse/util/stringutils.py
+++ b/synapse/util/stringutils.py
@@ -43,6 +43,14 @@ CLIENT_SECRET_REGEX = re.compile(r"^[0-9a-zA-Z\.=_\-]+$")
 #
 MXC_REGEX = re.compile("^mxc://([^/]+)/([^/#?]+)$")
 
+# https://spec.matrix.org/v1.13/appendices/#common-namespaced-identifier-grammar
+#
+# At least one character, less than or equal to 255 characters. Must start with
+# a-z, the rest is a-z, 0-9, -, _, or ..
+#
+# This doesn't check anything about validity of namespaces.
+NAMESPACED_GRAMMAR = re.compile(r"^[a-z][a-z0-9_.-]{0,254}$")
+
 
 def random_string(length: int) -> str:
     """Generate a cryptographically secure string of random letters.
@@ -68,6 +76,10 @@ def is_ascii(s: bytes) -> bool:
     return True
 
 
+def is_namedspaced_grammar(s: str) -> bool:
+    return bool(NAMESPACED_GRAMMAR.match(s))
+
+
 def assert_valid_client_secret(client_secret: str) -> None:
     """Validate that a given string matches the client_secret defined by the spec"""
     if (