You can use
^(.*?)(?:(\[RESP\])(.*))?$
See the regex demo. Details:
^ - start of string(.*?) - Group 1: any zero or more chars other than line break chars as few as possible(?:(\[RESP\])(.*))? - an optional sequence of:
(\[RESP\]) - Group 2: [RESP](.*) - Group 3: any zero or more chars other than line break chars as many as possible$ - end of string.