ExprParser

Simple, but flexible expression parser for Delphi.

There is often a point in the lifetime of a project, where you think, that you absolutely need scripting abilities, because you need a fast way to update the workflow of your application. But sometimes is a script engine a little bit oversized. In my cases it was completely sufficient to publish some context vars, bind some functions and then dynamicly evaluate through a simple expression. The expression can be modified outside without the need for a recompile. Maybe it is also useful for your needs.

ExprParser was forked from JvExprParser https://github.com/project-jedi/jvcl/blob/master/jvcl/run/JvExprParser.pas.

Since then it has gained some improvements:

• Speed improvements
• Short-circuit evaluation for boolean expression was introduced. It can be deactivated through TExprParser.FullBooleanEvaluation := True;
• The parser was unable to parse float numbers on non-English systems
• Hidden classes was moved from implementation to interface section. This enables it, to traverse the execution tree. This ability is also used in the contained TestSuite project.
• A lot of cases are now covered by unit tests

Example

Create a console application, add the ExprParser unit to the project and insert the following code

var
  Expr: TExprParser;
begin
  try
    Expr := TExprParser.Create;
    try
      Expr.OnGetVariable :=
        function(Sender: TObject; const VarName: string; var Value: Variant): Boolean
        begin
          if VarName = 'A' then
          begin
            Value := 156;
            Result := True;
          end
          else
            Result := False; // No other vars available
        end;

      // The following 2 lines demonstrate the sense of the efforts
      if Expr.Eval('(A * A) - (A * .25)') then
        Writeln('Result: ' + string(Expr.Value));
    finally
      Expr.Free;
    end;
  except
    on E: Exception do
      Writeln(E.ClassName, ': ', E.Message);
  end;

  Readln;
end.

The output will be:

Result: 24297