Searching for keywords in a file

(second time posting this year in ask a pro :)/>/>)

If i'm making a program and it contains code like:

echo "Hello world!"
pause

How can I split up the program and replace echo "Hello world!" with:

print("Hello world!")

And pause with:

print("Press any key to continue.")
os.pullEvent("key")

Like this:

file = fs.open("example", "r")
code = file.readAll()
file.close()
--search for "pause" and "echo" in "code"
execute = fs.open("executer", "w")
execute.write(code)
execute.close()
shell.run("executer")
fs.delete("executer")

http://lua-users.org/wiki/StringLibraryTutorial

Look for string.sub and string.find