How to find a string with missing fragments?

I'm building a chatbot in C# using AIML files, at the moment I've this code to process:

<aiml>
    <category>
        <pattern>a * is a *</pattern>
        <template>when a <star index="1"/> is not a <star index="2"/>?</template>
    </category>
</aiml>

I would like to do something like:

if (user_string == pattern_string) return template_string;

but I don't know how to tell the computer that the star character can be anything, and expecially that can be more than one word! I was thinking to do it with regular expressions, but I don't have enough experience with it. Can somebody help me? :)