I'm developing one at https://git.sr.ht/~remexre/rtf -- it's nowhere near ready for general use, but idiomatic code looks like:

    : parse-digit ( u -- u -1 | 0 0 )
      { DUP $30 $3a WITHIN } { $30 - TRUE }
      { DUP $41 $5b WITHIN } { $37 - TRUE }
      { DUP $61 $7b WITHIN } { $57 - TRUE }
      { ." parse-digit 1: " TODO } 3 CASE ;
    : add-digit ( u u -- u flag )
      ." add-digit 0: " TODO
      ;
    : parse-digits-0 ( u addr len -- u flag )
      { OVER B@ -ROT { parse-digit } 2DIP ROT
        { { SWAP base B@ * + } 2DIP 1 /STRING parse-digits-0 } WHEN
      } { DROP TRUE } ?COND ;
    : parse-digits ( u addr len -- u flag )
      { parse-digits-0 } { DROP FALSE } ?COND ;
    : parse-prefix-if-exists ( addr len -- addr len 0|1|2 )
      DUP {
        OVER B@
        { DUP $2d = } { DROP 1 /STRING 1 }
        { DUP $7e = } { DROP 1 /STRING 2 }
        { DROP 0 } 2 CASE
      } { 0 } COND ;
    : parse-base-if-exists ( addr len -- addr len base )
      DUP {
        OVER B@
        { DUP $23 = } { DROP 1 /STRING #10 }
        { DUP $24 = } { DROP 1 /STRING $10 }
        { DUP $25 = } { DROP 1 /STRING %10 }
        { DUP $5e = } { DROP 1 /STRING ^10 }
        { DROP 2DUP $3a SCAN DUP } {
          1 /STRING { NIP - 1- 10 BASE B! 0 -ROT parse-digits } 2KEEP
          ROT { ROT } {
            ." parse-base-if-exists 1: " TODO
          } COND
          \ ." parse-base-if-exists 2: " TODO
        }
        { 2DROP base B@ } 5 CASE
      } { base B@ } COND ;
    : with-saved-base ( x*i { x*i -- x*j } -- x*j )
      base B@ SWAP DIP base B! ;
    : apply-prefix ( u 0|1|2 -- n )
      { } { NEGATE } { INVERT } 3 SWITCH ;
    : parse-number ( addr len -- u -1 | 0 0 )
      { parse-base-if-exists base B! parse-prefix-if-exists
        { 0 -ROT parse-digits } DIP
        SWAP { apply-prefix TRUE } { DROP FALSE } COND
      } with-saved-base ;

(This being an incomplete rewrite of the bootstrapped text interpreter's number parser; as I said, nowhere near ready.)