First attempt. Has a lag issue due to the async-clear :(

src/SlashCommands.js

@@ -81,6 +81,8 @@ class Command {
     }
 
     run(roomId, args) {
+        // if it has no runFn then its an ignored/nop command (autocomplete only) e.g `/me`
+        if (!this.runFn) return;
         return this.runFn.bind(this)(roomId, args);
     }
 
@@ -918,12 +920,12 @@ export function processCommandInput(roomId, input) {
     input = input.replace(/\s+$/, '');
     if (input[0] !== '/') return null; // not a command
 
-    const bits = input.match(/^(\S+?)( +((.|\n)*))?$/);
+    const bits = input.match(/^(\S+?)(?: +((.|\n)*))?$/);
     let cmd;
     let args;
     if (bits) {
         cmd = bits[1].substring(1).toLowerCase();
-        args = bits[3];
+        args = bits[2];
     } else {
         cmd = input;
     }
@@ -932,11 +934,8 @@ export function processCommandInput(roomId, input) {
         cmd = aliases[cmd];
     }
     if (CommandMap[cmd]) {
-        // if it has no runFn then its an ignored/nop command (autocomplete only) e.g `/me`
-        if (!CommandMap[cmd].runFn) return null;
-
         return CommandMap[cmd].run(roomId, args);
-    } else {
-        return reject(_t('Unrecognised command:') + ' ' + input);
     }
+    return null;
+    // return reject(_t('Unrecognised command:') + ' ' + input);
 }

src/components/views/rooms/SendMessageComposer.js

@@ -43,6 +43,9 @@ import ContentMessages from '../../../ContentMessages';
 import {Key} from "../../../Keyboard";
 import MatrixClientContext from "../../../contexts/MatrixClientContext";
 
+const SEND_ANYWAY = Symbol("send-anyway");
+const UNKNOWN_CMD = Symbol("unknown-cmd");
+
 function addReplyToMessageContent(content, repliedToEvent, permalinkCreator) {
     const replyContent = ReplyThread.makeReplyMixIn(repliedToEvent);
     Object.assign(content, replyContent);
@@ -194,7 +197,12 @@ export default class SendMessageComposer extends React.Component {
         return false;
     }
 
-    async _runSlashCommand() {
+    /**
+     * Parses and executes current input as a Slash Command
+     * @returns {Promise<Symbol|void>} UNKNOWN_CMD if the command is not known,
+     *  SEND_ANYWAY if the input should be sent as message instead
+     */
+    async _tryRunSlashCommand() {
         const commandText = this.model.parts.reduce((text, part) => {
             // use mxid to textify user pills in a command
             if (part.type === "user-pill") {
@@ -236,16 +244,38 @@ export default class SendMessageComposer extends React.Component {
             } else {
                 console.log("Command success.");
             }
+        } else {
+            const QuestionDialog = sdk.getComponent("dialogs.QuestionDialog");
+            // unknown command, ask the user if they meant to send it as a message
+            const {finished} = Modal.createTrackedDialog("Unknown command", "", QuestionDialog, {
+                title: _t("Unknown Command"),
+                description: _t("Unrecognised command: ") + commandText,
+                button: _t('Send as message'),
+                danger: true,
+            });
+            const [sendAnyway] = await finished;
+            return sendAnyway ? SEND_ANYWAY : UNKNOWN_CMD;
         }
     }
 
-    _sendMessage() {
+    async _sendMessage() {
         if (this.model.isEmpty) {
             return;
         }
+
+        let shouldSend = true;
+
         if (!containsEmote(this.model) && this._isSlashCommand()) {
-            this._runSlashCommand();
-        } else {
+            const resp = await this._tryRunSlashCommand();
+            if (resp === UNKNOWN_CMD) {
+                // unknown command, bail to let the user modify it
+                return;
+            }
+
+            shouldSend = resp === SEND_ANYWAY;
+        }
+
+        if (shouldSend) {
             const isReply = !!RoomViewStore.getQuotingEvent();
             const {roomId} = this.props.room;
             const content = createMessageContent(this.model, this.props.permalinkCreator);
@@ -259,6 +289,7 @@ export default class SendMessageComposer extends React.Component {
                 });
             }
         }
+
         this.sendHistoryManager.save(this.model);
         // clear composer
         this.model.reset([]);

src/i18n/strings/en_EN.json

@@ -200,7 +200,6 @@
     "Sends the given message coloured as a rainbow": "Sends the given message coloured as a rainbow",
     "Sends the given emote coloured as a rainbow": "Sends the given emote coloured as a rainbow",
     "Displays list of commands with usages and descriptions": "Displays list of commands with usages and descriptions",
-    "Unrecognised command:": "Unrecognised command:",
     "Reason": "Reason",
     "%(targetName)s accepted the invitation for %(displayName)s.": "%(targetName)s accepted the invitation for %(displayName)s.",
     "%(targetName)s accepted an invitation.": "%(targetName)s accepted an invitation.",
@@ -1077,6 +1076,9 @@
     "Server error": "Server error",
     "Command error": "Command error",
     "Server unavailable, overloaded, or something else went wrong.": "Server unavailable, overloaded, or something else went wrong.",
+    "Unknown Command": "Unknown Command",
+    "Unrecognised command: ": "Unrecognised command: ",
+    "Send as message": "Send as message",
     "Failed to connect to integration manager": "Failed to connect to integration manager",
     "You don't currently have any stickerpacks enabled": "You don't currently have any stickerpacks enabled",
     "Add some now": "Add some now",