lua has decent string lib in the stdlib, you can also use 3rd party libraries such as https://github.com/stein197/lua-string, I'm not familiar with tcl, can you elaborate why tcl is much better than lua in string manipulations?
Tcl is built around string manipulation, it's the main and, for a long time, only data type. It has regex built in, a full set of string manipulation functions, and an interpolation primitive.
It's not about what's better. They are both full blown programming languages and you can implement any behavior you care to. In lua you have to care to implement string functions if you're going to do anything complex with strings. Tcl comes with them. That's all.