BUG in tclsh "BRE" is not accepted

docu
        An ARE may begin with embedded options: a sequence (?xyz) (where xyz is one or more alphabetic characters) specifies
        options affecting the rest of the RE. These supplement, and can override, any options specified by  the  application.
        The available option letters are:
          b  rest of RE is a BRE
 > echo 'regexp {(?b\<otto\>)} "hallo otto home"' | tclsh
couldn't compile regular expression pattern: invalid embedded option
→ don't know HOWTO activate a BRE
(the reason for this question is that BRE is the default RE of the "grep" command and
if TCL and GREP work together they COULD share the BRE)