Parse/Replace Text that contains conditional placeholders

Looking for a little advice or knowledge of 3rd party libraries that may help with this.

For a user-generated set of template text, such as those found below, how would you go about (automatically as possible) parsing and replacing to get the logical output?

The overall concept: the user will be inputting information about a sale into my API. I then need to mail merge the sale properties (name, address, features of what they bought) into a plain text, contract of sorts. The contract is user-editable, and their responsibility to populate and maintain. The contract has many conditional blocks of text, and the conditions are met/judged based on the sale's data, and the permutation of sale property and conditions vary by contract and user.

I've already built a couple C# factories for this that work fine. The factories use a combination of RegEx matching and good ol' substring and splitting, but with each iteration I'm finding more flexibility deficiencies. For example, if there's a predefined placeholder like

{{if sale.type == abc}} print this {{else if sale.type == 123}} print that {{else}} print foo {{end}}

and the user decides they don't need the "else if", remove it, and just use

{{if sale.type == abc}} print this {{else}} print foo {{end}}

the combination of RegEx matching, substring, splitting, etc starts falling apart.

Additionally, I'm considering using reflection instead of predefining all the possible conditions based on the properties of the sale. Instead, I'd just give the user a list of the property names, and allow them to construct their own conditional placeholders based on whatever property(s) of the sale they want.

I'd imagine that must be templating engines out there that can do this, but I've yet to run into one that is current, relevant, or otherwise - but maybe I'm missing something or ignorant of the terminology.

Ideally, I'd like this templating to support things like

• basic, replace placeholders
• conditional branching (if/elseif/else, in/contains)
• nesting of conditions
• either use to use out of the box, or extensible so that I have a user of moderate ability use it.

Thanks! Open to any/all suggestions, comments, and idea.

Template Examples:

#1-basic placeholder and an if/else/end

Hi. Today is {{datetime}}.
{{if obj.isReady == true}} I see you are ready. {{else}} I'm sorry you're not ready. {{end}}

#2-an if in/else/end

{{if obj.Color IN [blue,red]}} I love that color too. {{else}} Eh, not my favorite. {{end}}

#3-an if/else if/else/end

{{if obj.Type == rock}} ROLL! {{else if obj.Type == paper}} COVER! {{else}} SNIP, SNIP! {{end}}

#4-nested conditions
{{if obj.Happy == true}} Great you're happy! {{if obj.Season == summer}} So warm! {{else}} Burr! {{end}} {{else}} I'm sorry you're not happy. {{end}}

You can use various methods, one of them is RazorEngine.

using RazorEngine;
using RazorEngine.Templating;
using System;
using System.Text.RegularExpressions;

class Program
{
    static void Main(string[] args)
    {
        var template = @"Hi. Today is @DateTime.Now. @if (Model.IsReady) { <text>I see you are ready.</text> } else {<text>I'm sorry you're not ready.</text>}";
        var model = new { IsReady = true };
        var result = Engine.Razor.RunCompile(template, "template", null, model);
		result = Regex.Replace(result, "<.*?>", string.Empty);
        Console.WriteLine(result);
    }
}

The problem with this is just that you need to hack the errors when RazorEngine expects html tags. Not sure if this is the most efficient way also.

https://dotnetfiddle.net/MfTo8j