Question

Well, here is my problem: I have an application that uses a custom Javascript implementation,
but no support for Regular Expressions.

However, I’d like to be able to parse templates nevertheless; preferably using C++.

A template might look like this (ASP-style template):

<% var foo = someFunction("with a string");
   var bar =  anotherFunction(["with", "an", "array"]); %>

<b>This is html, and this is a variable: <%= bar %></b>

<% if(foo) { %>
    <b> foo is 'true'</b>
<% } else { %>
    <b> foo is 'false'. terrible. </b>
<% } %>

So the general structure is pretty simple (and I’d assume, relatively parseable).

My question is, Is it possible to parse such a template with a while() loop, going through each character, instead of using regular expressions?

And since my attempts to do that failed horribly, how could it be done?

Thank you!

Answer 1

Have you thought about using a Finite State Machine? Here are some links for your to look at.

• StackOverflow – Finite State Machine parser
• StackOverflow – FSM Design and ownership
• Simple C++ FSM

In short: FSM consists of finite number of states and transitions between these states. So the process of your parsing might be expressed as follows (pseudocode):

myFSM = new FSM( /* states, transitions */ );
// now your FSM is at initial state.

while not end of file {       
  switch (myFSM->currentState) {   
    case 'IF':
      // Does current line contain closing if? or else? If so, do a transition
      // to state that grabs everything in if construct 
      ...
    case 'TEXT': 
      // Lines do not have any lexical constructs, and we are outside any blocks
      ...
    ...

  }
}

Of course, this is hugely simplified. A real parser would look different. But I hope you got an idea.