Question

I have a text template with placehoders that I parse in order to replace placeholders
with real values.

Text Template:

Name:%name%
Age:%age%

I use StringBuilder.Replace() to replace placeholders

sb.Replace("%name%", Person.Name);

Now I want to make more advanced algorithm. Some lines of code are conditional. They
have to be either removed completely of kept.

Text Template

Name:%Name%
Age:%age%
Employer:%employer%

The line Employer should appear only when person is employed (controlled by boolean variable Person.IsEmployed).

UPDATE: I could use open/close tags. How can find text between string A and B?
Can I use Regex? How?

Answer 1

Your current templating scheme isn’t robust enough – you should add more special placeholders, like this for example:

Name:%Name%
Age:%age%
[if IsEmployed]
Employer:%employer%
[/if]

You can parse out [if *] blocks using a regex (not tested):

Match[] ifblocks = Regex.Match(input, "\\[if ([a-zA-Z0-9]+)\\]([^\\[]*)\\[/if\\]");
foreach(Match m in ifblocks) {
    string originalBlockText = m.Groups[0];
    string propertyToCheck = m.Groups[1];
    string templateString = m.Groups[2];

    // check the property that corresponds to the keyword, i.e. "IsEmployed"

    // if it's true, do the normal replacement on the templateString
    // and then replace the originalBlockText with the "filled" templateString

    // else, just don't write anything out
}

Really, though, this implementation is full of holes… You may be better off with a templating framework like another answer suggested.