(sub)string comp

[spir <denis.spir@...>]

Hello,

I have the impression to miss an obvious point (again). In all kinds of string 
processing (and even more in parsing), we constantly need to check whether a 
given substring is present at a given index in a string. I don't see any other 
way to do that in Lua than:
   if (str:sub(i, i-1+sub:len()) == sub) then ... end
That is, to make a new (sub)string and compare it with the original one. Hum. Am 
I overlooking a trivial & efficient solution?

Denis