diff --git a/src/User.c b/src/User.c
index 9e36239..2777d79 100644
--- a/src/User.c
+++ b/src/User.c
@@ -31,6 +31,10 @@
 #include <Cytoplasm/Int64.h>
 #include <Cytoplasm/UInt64.h>
 
+#include <Parser.h>
+
+#include <ctype.h>
+
 #include <string.h>
 
 struct User
@@ -882,51 +886,88 @@ finish:
     return arr;
 }
 
+static int
+UserIdParseLocal(Parser *p, char **ret)
+{
+    int c;
+    if (!p || !ret)
+    {
+        return 0;
+    }
+
+    ParserStartCopy(p);
+    /* Always assume the general case here: every ASCII char, 
+     * minus ':'. */
+    while (isascii((c = ParserGetc(p))) && c != ':')
+    {
+        /* Do nothing. */
+    }
+    if (c != EOF)
+    {
+        ParserUndo(p);
+    }
+    *ret = ParserEndCopy(p);
+    return 1;
+}
+static int
+UserIdParseServer(Parser *p, char **ret)
+{
+    int c;
+    if (!p || !ret)
+    {
+        return 0;
+    }
+
+    ParserStartCopy(p);
+    /* Allow *every* ASCII character. */
+    while (isascii((c = ParserGetc(p))))
+    {
+        /* Do nothing. */
+    }
+    if (c != EOF)
+    {
+        ParserUndo(p);
+    }
+    *ret = ParserEndCopy(p);
+    return 1;
+}
+
 UserId *
 UserIdParse(char *id, char *defaultServer)
 {
     UserId *userId;
+    Parser *p;
 
     if (!id)
     {
         return NULL;
     }
 
-    id = StrDuplicate(id);
-    if (!id)
-    {
-        return NULL;
-    }
-
+    p = ParserCreate(id);
     userId = Malloc(sizeof(UserId));
+    userId->localpart = NULL;
+    userId->server = NULL;
+
     if (!userId)
     {
         goto finish;
     }
 
-    /* Fully-qualified user ID */
-    if (*id == '@')
+    /* Fully-qualified user ID.
+     * TODO: Generalise this for parsing namespaced IDs, 
+     * like room aliases and IDs. */
+    if (ParserExcept(p, '@'))
     {
-        char *localStart = id + 1;
-        char *serverStart = localStart;
-
-        while (*serverStart != ':' && *serverStart != '\0')
-        {
-            serverStart++;
-        }
-
-        if (*serverStart == '\0')
+        if (!UserIdParseLocal(p, &userId->localpart)    || 
+            !ParserExcept(p, ':')                       || 
+            !UserIdParseServer(p, &userId->server))
         {
+            if (userId->localpart)
+            {
+                Free(userId->localpart);
+            }
             Free(userId);
-            userId = NULL;
-            goto finish;
         }
-
-        *serverStart = '\0';
-        serverStart++;
-
-        userId->localpart = StrDuplicate(localStart);
-        userId->server = StrDuplicate(serverStart);
     }
     else
     {
@@ -942,7 +983,7 @@ UserIdParse(char *id, char *defaultServer)
     }
 
 finish:
-    Free(id);
+    ParserEnd(p);
     return userId;
 }