algorithm - Simple string replacement set of rules -

I have an application where users have been bundled with a bunch of text boxes and a bunch of objects I will show these items. Just need to set up a person object, for which you need to enter the name and LastName properties.

Now I want to implement the global variable that users are able to assign values, or values ​​will change during the execution of the program. And I want the users to be able to use them when filling the properties of an object. My first idea was to select a special character that would mark the beginning of a variable name, and then allow the user character to use himself twice to represent himself. For example, I have a global variable named Macking . Again, say that I am the symbol marking the beginning of a symbol % . The user will then be able to enter the name of a person's last name in the form of "Mac. Miking", then I can replace it using the value of Macking . If the value of Macking is "Donald", then finally "Mac Donald" will be created.

With this problem, if I have a variable named that and the other Hello and the user will enter "% Hello" string I Do not know which variable needs to be replaced. I can change my rules, for example, use the "%" symbol to mark the beginning and end of the variable name. But I'm not sure that this would be the cause of any other problem.

What will be the simplest possible set of rules to achieve in such a way that the user will be able to represent every possible string without any ambiguity? Ideally, variable names can contain any characters, but I can restrict their names to the set set of characters. Your approach to mark both the beginning and end of

% What is the problem if the input string is % foo %% times% ? Do I get the value of foo and the value of bar ? Or do I get the value of foo% bar ? (Definitely if the << code>% variable names are not allowed, this is not a problem.)

The easiest way I can think of to avoid this problem I am using one to start the symbol for the beginning and the second for the end (like # ) should avoid any ambiguity if the user has a # Wants it, then it avoids it: % # . There is no problem with this, because there is no empty variable name (at least I do not expect).