Problem Statement

Many languages have powerful and useful (binary safe) string manipulation functions, while others don't, making it harder for these languages to accomplish some tasks. This task is about creating functions to handle binary strings (strings made of arbitrary bytes, i.e. byte strings according to Wikipedia) for those languages that don't have built-in support for them. If your language of choice does have this built-in support, show a possible alternative implementation for the functions or abilities already provided by the language. In particular the functions you need to create are:

Possible contexts of use: compression algorithms (like LZW compression), L-systems (manipulation of symbols), many more.

Let's start with the solution:

cmp_str(u,v)=u==v
copy_str(v)=v \\ Creates a copy, not a pointer
append_str(v,n)=concat(v,n)
replace_str(source, n, replacement)=my(v=[]);for(i=1,#source,v=concat(v,if(source[i]==n,replacement,source[i]))); v

u=[72, 101, 108, 108, 111, 44, 32, 119, 111, 114, 108, 100];
v=[];
cmp_str(u,v)
w=copy_str(v)
#w==0
append_str(u,33)
u[8..12]
replace_str(u,108,[121])
concat(v,w)