17.6.2.2375
UNESCAPE
 
STRING EXT
 
( c-addr1 u1 c-addr2 -- c-addr2 u2 )

Replace each `%' character in the input string c-addr1 u1 by two `%' characters. The output is represented by c-addr2 u2. The buffer at c-addr2 shall be big enough to hold the unescaped string. An ambiguous condition occurs if the resulting string will not fit into the destination buffer (c-addr2).

See:
17.6.2.2255 SUBSTITUTE.

Implementation:
: UNESCAPE \ c-addr1 len1 c-addr2 -- c-addr2 len2
\ Replace each '%' character in the input string c-addr1 len1 with two '%' characters.
\ The output is represented by c-addr2 len2.
\ If you pass a string through UNESCAPE and then SUBSTITUTE, you get the original string.
   DUP 2SWAP OVER + SWAP ?DO
     I C@ [CHAR] % = IF
       [CHAR] % OVER C! 1+
     THEN
     I C@ OVER C! 1+
   LOOP
   OVER -
;
Testing:
Using subbuff, sub5 and sub6 from F.17.6.2.2255 SUBSTITUTE.

T{ sub6 subbuff UNESCAPE sub5 COMPARE -> 0 }T