fix text replace algorithm so it doesn't randomly delete text at the beginning of the message

Author: Tom Hutman

src/components/AutoCompleteTextarea/Textarea.js

@@ -203,12 +203,20 @@ class ReactTextareaAutocomplete extends React.Component {
 
     const textToModify = textareaValue.slice(0, selectionEnd);
 
-    const startOfTokenPosition = textToModify.search(
-      /**
-       * It's important to escape the currentTrigger char for chars like [, (,...
-       */
-      new RegExp(`\\${currentTrigger}${`[^\\${currentTrigger}${'\\s'}]`}*$`),
+    // It's important to escape the currentTrigger char for chars like [, (,...
+    const findTriggerRegExp = new RegExp(
+      `\\${currentTrigger}${`[^\\${currentTrigger}${'\\s'}]`}*`,
     );
+    // reverse the string so we can easily find the first index of the currentTrigger
+    const reversedString = textToModify.split('').reverse().join('');
+    // find the first instance of currentTrigger (-1 if not found)
+    const distanceFromEndOfString =
+      reversedString.search(findTriggerRegExp) + 1;
+    // if found, distract from the length of the string to revert the position back to an actual string index.
+    const startOfTokenPosition =
+      distanceFromEndOfString === -1
+        ? 0
+        : textToModify.length - distanceFromEndOfString;
 
     // we add space after emoji is selected if a caret position is next
     const newTokenString =