Language: C

main(){char s[99],*p;while(~scanf("%[ a-z.]\n",s)){while(!(strstr(s,"the")|strstr(s,"this")|strstr(s,"that")))for(p=s;*p;p++)*p>96&&*p<122?++*p:*p-122||(*p=97);puts(s);}return 0;}

Compiler output:
main.c:1:1: error: return type defaults to 'int' [-Wimplicit-int]
    1 | main(){char s[99],*p;while(~scanf("%[ a-z.]\n",s)){while(!(strstr(s,"the")|strstr(s,"this")|strstr(s,"that")))for(p=s;*p;p++)*p>96&&*p<122?++*p:*p-122||(*p=97);puts(s);}return 0;}
      | ^~~~
main.c: In function 'main':
main.c:1:29: error: implicit declaration of function 'scanf' [-Wimplicit-function-declaration]
    1 | main(){char s[99],*p;while(~scanf("%[ a-z.]\n",s)){while(!(strstr(s,"the")|strstr(s,"this")|strstr(s,"that")))for(p=s;*p;p++)*p>96&&*p<122?++*p:*p-122||(*p=97);puts(s);}return 0;}
      |                             ^~~~~
main.c:1:1: note: include '<stdio.h>' or provide a declaration of 'scanf'
  +++ |+#include <stdio.h>
    1 | main(){char s[99],*p;while(~scanf("%[ a-z.]\n",s)){while(!(strstr(s,"the")|strstr(s,"this")|strstr(s,"that")))for(p=s;*p;p++)*p>96&&*p<122?++*p:*p-122||(*p=97);puts(s);}return 0;}
main.c:1:29: warning: incompatible implicit declaration of built-in function 'scanf' [-Wbuiltin-declaration-mismatch]
    1 | main(){char s[99],*p;while(~scanf("%[ a-z.]\n",s)){while(!(strstr(s,"the")|strstr(s,"this")|strstr(s,"that")))for(p=s;*p;p++)*p>96&&*p<122?++*p:*p-122||(*p=97);puts(s);}return 0;}
      |                             ^~~~~
main.c:1:29: note: include '<stdio.h>' or provide a declaration of 'scanf'
main.c:1:60: error: implicit declaration of function 'strstr' [-Wimplicit-function-declaration]
    1 | main(){char s[99],*p;while(~scanf("%[ a-z.]\n",s)){while(!(strstr(s,"the")|strstr(s,"this")|strstr(s,"that")))for(p=s;*p;p++)*p>96&&*p<122?++*p:*p-122||(*p=97);puts(s);}return 0;}
      |                                                            ^~~~~~
main.c:1:1: note: include '<string.h>' or provide a declaration of 'strstr'
  +++ |+#include <string.h>
    1 | main(){char s[99],*p;while(~scanf("%[ a-z.]\n",s)){while(!(strstr(s,"the")|strstr(s,"this")|strstr(s,"that")))for(p=s;*p;p++)*p>96&&*p<122?++*p:*p-122||(*p=97);puts(s);}return 0;}
main.c:1:60: warning: incompatible implicit declaration of built-in function 'strstr' [-Wbuiltin-declaration-mismatch]
    1 | main(){char s[99],*p;while(~scanf("%[ a-z.]\n",s)){while(!(strstr(s,"the")|strstr(s,"this")|strstr(s,"that")))for(p=s;*p;p++)*p>96&&*p<122?++*p:*p-122||(*p=97);puts(s);}return 0;}
      |                                                            ^~~~~~
main.c:1:60: note: include '<string.h>' or provide a declaration of 'strstr'
main.c:1:75: error: invalid operands to binary | (have 'char *' and 'char *')
    1 | main(){char s[99],*p;while(~scanf("%[ a-z.]\n",s)){while(!(strstr(s,"the")|strstr(s,"this")|strstr(s,"that")))for(p=s;*p;p++)*p>96&&*p<122?++*p:*p-122||(*p=97);puts(s);}return 0;}
      |                                                            ~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~
      |                                                            |               |
      |                                                            char *          char *
main.c:1:161: error: implicit declaration of function 'puts' [-Wimplicit-function-declaration]
    1 | main(){char s[99],*p;while(~scanf("%[ a-z.]\n",s)){while(!(strstr(s,"the")|strstr(s,"this")|strstr(s,"that")))for(p=s;*p;p++)*p>96&&*p<122?++*p:*p-122||(*p=97);puts(s);}return 0;}
      |                                                                                                                                                                 ^~~~
main.c:1:161: note: include '<stdio.h>' or provide a declaration of 'puts'