identifiers: Fix MXC URI validation

crates/ruma-identifiers-validation/CHANGELOG.md

@@ -1,5 +1,13 @@
 # [unreleased]
 
+Bug fixes:
+
+- Allow underscores (`_`) when validating MXC URIs.
+  - They have always been allowed in [the spec][mxc validation spec]
+    in order to support URL-safe base64-encoded media IDs.
+
+[mxc validation spec]: https://spec.matrix.org/v1.9/client-server-api/#security-considerations-5
+
 Improvements:
 
 - Point links to the Matrix 1.9 specification

crates/ruma-identifiers-validation/src/mxc_uri.rs

@@ -18,8 +18,9 @@ pub fn validate(uri: &str) -> Result<NonZeroU8, MxcUriError> {
     let server_name = &uri[..index];
     let media_id = &uri[index + 1..];
     // See: https://spec.matrix.org/v1.9/client-server-api/#security-considerations-5
-    let media_id_is_valid =
-        media_id.bytes().all(|b| matches!(b, b'0'..=b'9' | b'a'..=b'z' | b'A'..=b'Z' | b'-' ));
+    let media_id_is_valid = media_id
+        .bytes()
+        .all(|b| matches!(b, b'0'..=b'9' | b'a'..=b'z' | b'A'..=b'Z' | b'-' | b'_' ));
 
     if !media_id_is_valid {
         Err(MxcUriError::MediaIdMalformed)