Language: C

char r[99],g[99],*G,d[99];main(c,i){for(;scanf("%s%s%s",r,g,d)>1;puts(G))for(i=k=0,G=g,c=*G++;*(G-1);)c=c==d[i]?r[i++]:(putchar(c),*G++);exit(0);}

Compiler output:
main.c:1:27: error: return type defaults to 'int' [-Wimplicit-int]
    1 | char r[99],g[99],*G,d[99];main(c,i){for(;scanf("%s%s%s",r,g,d)>1;puts(G))for(i=k=0,G=g,c=*G++;*(G-1);)c=c==d[i]?r[i++]:(putchar(c),*G++);exit(0);}
      |                           ^~~~
main.c: In function 'main':
main.c:1:27: error: type of 'c' defaults to 'int' [-Wimplicit-int]
main.c:1:27: error: type of 'i' defaults to 'int' [-Wimplicit-int]
main.c:1:42: error: implicit declaration of function 'scanf' [-Wimplicit-function-declaration]
    1 | char r[99],g[99],*G,d[99];main(c,i){for(;scanf("%s%s%s",r,g,d)>1;puts(G))for(i=k=0,G=g,c=*G++;*(G-1);)c=c==d[i]?r[i++]:(putchar(c),*G++);exit(0);}
      |                                          ^~~~~
main.c:1:1: note: include '<stdio.h>' or provide a declaration of 'scanf'
  +++ |+#include <stdio.h>
    1 | char r[99],g[99],*G,d[99];main(c,i){for(;scanf("%s%s%s",r,g,d)>1;puts(G))for(i=k=0,G=g,c=*G++;*(G-1);)c=c==d[i]?r[i++]:(putchar(c),*G++);exit(0);}
main.c:1:42: warning: incompatible implicit declaration of built-in function 'scanf' [-Wbuiltin-declaration-mismatch]
    1 | char r[99],g[99],*G,d[99];main(c,i){for(;scanf("%s%s%s",r,g,d)>1;puts(G))for(i=k=0,G=g,c=*G++;*(G-1);)c=c==d[i]?r[i++]:(putchar(c),*G++);exit(0);}
      |                                          ^~~~~
main.c:1:42: note: include '<stdio.h>' or provide a declaration of 'scanf'
main.c:1:66: error: implicit declaration of function 'puts' [-Wimplicit-function-declaration]
    1 | char r[99],g[99],*G,d[99];main(c,i){for(;scanf("%s%s%s",r,g,d)>1;puts(G))for(i=k=0,G=g,c=*G++;*(G-1);)c=c==d[i]?r[i++]:(putchar(c),*G++);exit(0);}
      |                                                                  ^~~~
main.c:1:66: note: include '<stdio.h>' or provide a declaration of 'puts'
main.c:1:80: error: 'k' undeclared (first use in this function)
    1 | char r[99],g[99],*G,d[99];main(c,i){for(;scanf("%s%s%s",r,g,d)>1;puts(G))for(i=k=0,G=g,c=*G++;*(G-1);)c=c==d[i]?r[i++]:(putchar(c),*G++);exit(0);}
      |                                                                                ^
main.c:1:80: note: each undeclared identifier is reported only once for each function it appears in
main.c:1:121: error: implicit declaration of function 'putchar' [-Wimplicit-function-declaration]
    1 | char r[99],g[99],*G,d[99];main(c,i){for(;scanf("%s%s%s",r,g,d)>1;puts(G))for(i=k=0,G=g,c=*G++;*(G-1);)c=c==d[i]?r[i++]:(putchar(c),*G++);exit(0);}
      |                                                                                                                         ^~~~~~~
main.c:1:121: note: include '<stdio.h>' or provide a declaration of 'putchar'
main.c:1:138: error: implicit declaration of function 'exit' [-Wimplicit-function-declaration]
    1 | char r[99],g[99],*G,d[99];main(c,i){for(;scanf("%s%s%s",r,g,d)>1;puts(G))for(i=k=0,G=g,c=*G++;*(G-1);)c=c==d[i]?r[i++]:(putchar(c),*G++);exit(0);}
      |                                                                                                                                          ^~~~
main.c:1:1: note: include '<stdlib.h>' or provide a declaration of 'exit'
  +++ |+#include <stdlib.h>
    1 | char r[99],g[99],*G,d[99];main(c,i){for(;scanf("%s%s%s",r,g,d)>1;puts(G))for(i=k=0,G=g,c=*G++;*(G-1);)c=c==d[i]?r[i++]:(putchar(c),*G++);exit(0);}
main.c:1:138: warning: incompatible implicit declaration of built-in function 'exit' [-Wbuiltin-declaration-mismatch]
    1 | char r[99],g[99],*G,d[99];main(c,i){for(;scanf("%s%s%s",r,g,d)>1;puts(G))for(i=k=0,G=g,c=*G++;*(G-1);)c=c==d[i]?r[i++]:(putchar(c),*G++);exit(0);}
      |                                                                                                                                          ^~~~
main.c:1:138: note: include '<stdlib.h>' or provide a declaration of 'exit'