Make an interpreter for yes!
JavaScript (ES6), 218 215 204 203 bytes
Takes the program string s and the input i in currying syntax (s)(i).
s=>i=>s.replace(/\w+/g,S=>(c=eval("[P()];P()+1;[s.shift()];1;0;s=[s.join``];P()-1;i;P()[0]?k.charCodeAt():String.fromCharCode(k)".split`;`[parseInt(S,35)%156%9])).map||s.push(c),s=[],P=_=>k=s.pop())&&P()
How?
We use the perfect hash function parseInt(S, 35) % 156 % 9 to convert the instruction S into an index in 0…8 and use this index to select the JS code to be executed:
instruction | base 35 -> dec. | % 156 | % 9 | JS code
------------+-----------------+-------+-----+---------------------------------------------
"yes" | 42168 | 48 | 3 | 1
"no" | 829 | 49 | 4 | 0
"what" | 1393204 | 124 | 7 | i
"sure" | 1238209 | 37 | 1 | P()+1
"nah" | 28542 | 150 | 6 | P()-1
"really" | 1439554619 | 35 | 8 | P()[0]?k.charCodeAt():String.fromCharCode(k)
"oh" | 857 | 77 | 5 | s=[s.join``]
"nope" | 1016414 | 74 | 2 | [s.shift()]
"yep" | 42165 | 45 | 0 | [P()]
The P function pops the last item off the stack s and loads it into k.
We prevent the result of some instructions from being pushed back onto the stack by testing if the .map() method is defined, that is, if the result is an array. The code for oh is returning an array by design and we force nope and yep to return arrays as well. Hence the syntax:
(c = eval("[code0];code1;...".split`;`[index])).map || s.push(c)
Test cases
let f =
s=>i=>s.replace(/\w+/g,S=>(c=eval("[P()];P()+1;[s.shift()];1;0;s=[s.join``];P()-1;i;P()[0]?k.charCodeAt():String.fromCharCode(k)".split`;`[parseInt(S,35)%156%9])).map||s.push(c),s=[],P=_=>k=s.pop())&&P()
console.log(f(`yes sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure really
yes sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure really
yes sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure really
yes sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure really
yes sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure really
yes sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure really
yes sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure really
yes sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure really
yes sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure really
yes sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure really
yes sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure really
yes sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure really
yes sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure really
oh`)())
console.log(f(`what`)(1234))
console.log(JSON.stringify(f(`no really`)()))
console.log(f(`what
yes sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure really
oh`)('code golf'))
console.log(f(`yes no nope`)())
console.log(f(`yes no yep`)())Röda, 256 bytes
f c,n{s=[];(c/`\W|_`)()|{|m|s+=#m-2 if[m=~"yes|no"];s[-1]+=#m*2-7 if[m=~"sure|nah"];s+=n if[m="what"];s=s[#m%3:#s-#m%2]if[m=~"nope|yep"];{s+=""s()|s[-1].=_}if[m="oh"];{t=s[-1]y=t..""a=t+0;a=a..""{s[-1]=ord(s)}if[#a>#y]else{s[-1]=chr(t)}}if[#m=6]}_;[s[-1]]}
Try it online!
Explanation
#variable returns the length of variable (if it is a string or an array).
f c,n{ /*declare a function f with arguments c and n*/
s=[]; /*initialise the stack*/
(c/`\W|_`) /*split the code on anything not [a-zA-Z0-9]*/
()| /*and push each of its values to the stream*/
{|m|...}_ /*for each element m in the stream, do:*/
s+=#m-2 if[m=~"yes|no"]; /* add 1 or 0 to the stack if m is "yes" or "no"*/
s[-1]+=#m*2-7if[m=~"sure|nah"];/* increment or decrement the top element if m is "sure" or "nah"*/
s+=n if[m="what"]; /* push input if m is "what"*/
s=s[#m%3:#s-#m%2] /* remove the first or last element of the stack
if[m=~"nope|yep"]; /* if m is "nope" or "yep" */
{ }if[m="oh"]; /* if m is "oh" do:*/
s+="" /* add an element to the stack*/
s()|s[-1].=_ /* for each element in s, concatenate that amount to the last element of the stack*/
{ }if[#m=6] /* if m is "really" (it's length is 6) do:*/
t=s[-1]y=t.."" /* get the last element of the stack*/
a=t+0;a=a.."" /* add 0 to it, if a is a number, this does nothing, otherwise this makes a longer by 1 character*/
{s[-1]=ord(s)}if[#a>#y] /* if a is longer than t (the last element is a char/string) replace it with its code point*/
else{s[-1]=chr(t)} /* otherwise, replace the last element with the char it represents*/
[s[-1]] /*finally output the top of the stack*/
05AB1E, 77 67 63 61 bytes
Að«Ã#vyÇO§}ðý•9ǝ×н}ÀÀÙ™Íð•650в"X ¾ I > < DdiçëÇ} J r\r \"#:.V
Assumes the program is on the top of the stack.
Try it online!